Question for the experts.

Hi everyone! I'm proud to say that I lifted weights about 3 times last week and did cardio about 4 mornings last week. I'm trying to do better than that, but my hubby and I like to work out together and our schedules have been crazy. Here's my question....I worked out my triceps, biceps, and calves yesterday after work. I noticed that I seemed to struggle toward the end with the higher weights / lower reps and that I hadn't last time I worked those muscles. I do a modified "body for life" workout, at least as far as going up in weight and down in reps. I break my workouts into three to work the whole body instead of two because I usually have a limited time frame in which to work out. I've just never noticed the same weights feeling heavier than the previous workout before. And I had ample days between for rest and growth. Any ideas would be welcome? Thanks! Jennifer P.S. My endocrinologist wants to tweek my thyroid meds, so he won't let me get pregnant for at least 3 more months. I'm disappointed, but at least that's 3 more months to get my body in tip top shape.