It is always tough to tell how a reliever is doing. Their entire season is a small sample size. The best relievers pitch less than half as many innings as a starting pitcher. So, breaking it down further to one month could be even more foolhardy. One bad outing can ruin a reliever's ERA. Look at Tom Gorzelanny. He had one rough outing in the beginning of the season against the Astros where he gave up six runs, but that one outing is enough to make his 3.64 ERA enough to not fully explain how good and how important Gorzelanny has been to the Nationals bullpen.
Small sample size or not the Nationals bullpen was not good in July. They have the seventh worst bullpen ERA over that time at 4.73. Clippard, H-Rod, and Mattheus all have ERA's over 5.00 while Storen and Stammen both have ERA's in the 4.00. The most innings pitched in July were the 15 1/3 of Stammen, but if the Nats bullpen continues to falter like this things might get even tighter as the Braves are starting to roll.