HOUSTON (AP) - J.A. Happ pitched six strong innings, hit the first home run of his career and then watched the Astros bullpen blow another lead Sunday at Minute Maid Park.

Happ recovered after walking the bases loaded in the first inning and allowed two hits over six innings and hit a 333-foot homer in the fourth inning for a 2-0 lead over the Arizona Diamondbacks.

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It wasn't enough.

Xavier Nady hit a two-run double in the eighth inning, helping the Diamondbacks rally for a 4-2 victory and a series sweep of the Astros. Arizona has won six straight road games and 14 of 16 overall to move into first in the NL West.

Nady's go-ahead double off Jeff Fulchino (1-3) gave the Diamondbacks their first lead and Juan Miranda followed with an RBI single for a two-run cushion.

The Astros have blown 13 of their 20 save opportunities in 2011.

``Tough series for us,' Happ said. ``Getting swept at home isn't a very good thing, so we are going to try to regroup here and have a good road trip starting in Chicago.'

Astros manager Brad Mills said he still has confidence in his bullpen.

``That is absolutely not an issue,' said Mills. ``They have been throwing the ball really well lately, and we've got all the confidence in the world in them. (Fulchino) has done a real good job for us this year, getting a lot of big outs for us.'

Happ walked the bases loaded with one out in the first inning, but didn't allow a hit until Willie Bloomquist lined a single into center field with two outs in the fifth.

After loading the bases in the first, Happ retired the next 12 batters before Bloomquist's hit.

``I talked with (Houston pitching coach Brad) Arnsberg after that first inning a little bit,' added Happ. ``I don't know if it was just kind of a mechanical issue or what. I got my stride length down a little bit and was landing more where I needed to land instead of opening up a little bit. That might have been the difference.'

Happ hit a solo homer on the first pitch from Josh Collmenter in the fifth inning, the first of his career, giving Houston a 2-0 lead.

Happ, who said he did not get the home run ball, walked four and struck out eight.

``Happy did a great job settling down after that first inning with three walks,' Mills said. ``He really came back strong right there and got out of that jam. And then, of course, he hit that homer that gave us that second run. You can't say enough about how he threw the ball.'

Collmenter pitched six innings for Arizona, allowed four hits, walked one and struck out five. Collmenter didn't allow a hit until Jeff Keppinger led off the fourth with a double into the left field corner. He went to third on a single by Hunter Pence.

With Brett Wallace at the plate, umpire D.J. Reyburn called a balk on Collmenter, sending Keppinger home with the first run and Pence to second base.

Gibson came out to dispute the call and was immediately ejected.

Wallace walked but Chris Johnson grounded out and Clint Barmes struck out to hold Houston's lead to 1-0.

``He located well today,' said Barmes referring to Collmenter. ``And I think more than anything, it's where he's spotting the ball and his location was pretty effective. He's got a really good changeup. He's got a really, really fast arm speed on his changeup, which is makes it pretty good.'

Justin Upton tripled to the center field wall to lead off the sixth inning and scored on Chris Young's sacrifice fly.

Pence extended his hitting streak to 10 games. Keppinger has hit safely in the two games he's played this season since returning from the disabled list on Friday. Keppinger was 2 for 5 on Friday.

Aaron Heilman (4-0) struck out the side in the seventh for the victory.

J.J. Putz pitched the ninth for his 16th straight save to start the season. It is the longest current streak in the major leagues.

NOTES: Collmenter has the Arizona franchise record with 15consecutive scoreless innings to begin his career as a starter. Kelly Johnson has a five-game hitting streak, including a 4-for-5 performance Saturday night. Miranda has hit safely in eight of his last nine games. He hit two two-run homers in Friday night's 7-6 Arizona victory.