I just started bodybuilding about two to three weeks ago and I feel like I was gaining some muscle. Now it seems I'm back to square one. It's not like I gained a lot, but what I did gain, I've lost. Why is this happening when I am keeping up with my routine?
Bodybuilding is a long term endeavor, two to three weeks is nothing in the world of bodybuilding. If you'd said two to three years, then it would be a different matter.
Whatever you're doing, carry on doing it for at least 6 months, then see where you are with it.
That being said, if you want to gain muscle (which I assume is what you mean by bodybuilding) check your diet and recovery, if either of those aren't up to par, then it'll stifle your gains.
Its a long journey. For natural lifter, being very strong and muscular takes a good couple of years.
Since you are a new lifter progressive overload works fine for a few months, say 6 months. Then you can change program and still continue adding weights.
The last but not least, if you are eating too much it may cause belly fat that changes your appearence.