94 Altima Oil

I have a 94 Altima with the 2.4L Auto with 143xxx miles. I had a problem with my PCV valve being clogged, thus leading to oil backing up in to my distributor. I had the vehicle repaired and haven't had any more problems out of it. I had it repaired about a month ago & had an oil change performed at the same time. About 2 weeks ago I checked all my fluid levels and my oil was about a 1/2 quart low but was not still in the safe mark. I just got done checking all my fluid levels again and now my oil was 1 quart low, so I added a quart to it. This is the first time I've ever had to add any oil between oil changes and never had a problem with it leaking or being low before. Does anyone have any suggestions on whether me having to add oil this time was because it just worked it self low (once time thing) or if it has a leak. If someone believes it leaking, do you think its likely to be something unrelated to the repairs, or have to do with the repairs that were done, in which case I could take it back to the shop that did the repairs & find out what the hell's going on? I will be keeping a close eye on the oil to see if it leaks anymore & I have not noticed any oil leaks. Please help me out & give me your opinion.