Yes. Your body may simply be adjusting.
I always get really sick from test flu 3-4 weeks in. Every time, like clockwork. It is bad test flu too. I usually have to take a day off and rest. Test affects your immune system which can slow you down.
Also, test can affect sleep patterns. I have a hard time sleeping as it is, on 400mg of test I sometimes get 4-5 hours a night (try to catch up on the weekends) which simply isn't enough for me to recuperate from my workouts. I have been drained the past few weeks and I should be jacked because I am on cycle.
Look at your lifestyle and see if there are other things going on, stress, sleep, test flu, it could be a combination of factors.
Sometimes it doesn't hurt to get a blood panel done during cycle (skip the test tests because it will be through the roof). I just read in MD that a small percentage of people get severe elevated red blood cell counts when on cycle and this can cause problems. See what is going on inside of you. It doesn't hurt and online tests are cheap, probably under $200. I head to a local LabCorp and then get my results in 3-4 days.