Sing got it on the head.
I personally felt my first few cycles within days and they were long esters. I was probably lower end of the natural test levels so that had to have an influence. Literally within hours of your injection, short or long ester, the levels of that hormone go up in your blood. So if your blood levels go up I am willing to bet some of us are sensitive enough to feel or notice it. That doesn’t mean the anabolic effects are going to kick in right away, that takes time for the levels to build up to the threshold along with the necessary continued androgen receptor stimulation to see results. Even long esters start doing stuff within hours it’s just not enough to notice in regards to gains and such. Think about if you started ingesting a carcinogen that is going to start doing stuff but it might take years to turn into full blown cancer with tumors.
If you or anyone is interested go look up the steroidcalc app and website. Even just a simple 200 mg test e shot will release more than double the high end of natural test levels the very first day after that shot. Double the natural amount is going to start stimulating stuff but it takes a little time for the levels to get high enough to see significant results. We still have to get to that “anabolics threshold” to see the lovely 20 lbs of lean mass gained.
So to the OP yeah you could be experiencing placebo effects or you could be sensitive to masteron or just DHTs in general, either way I say go take advantage of it.