Building Strength

So...I'm slowly getting back into things (swimming, running, biking)...just getting in shape in general. I've been doing push-ups in the morning (15 on the perfect push-up/ 20 with my legs on the yoga ball) and lifting (5lb weights, 3x12 Flys, forwards, backs) at night. I felt a lot of strength improvement over the first 2-3 weeks (I've increased the # of push-ups from 10/15) but now...not so much. In fact, I feel like they are much harder now than they were last week at the same repetitions. So my thoughts on the issue is that I am not getting enough nutritional intake to support strength growth. Anyone have suggestions? thoughts? ideas?