That's likely your menstruation cycle! Trans women have observed and experienced menstruation since HRT became a thing
We have all the parts needed to experience menstruation, just less of some of the tissues involved, so typically we experience weaker or even not noticeable periods. Some trans women do have very strong periods including period shits, cramps, and large mood swings. It's also possible that how we take HRT affects how noticeable the period is. I'ts not well studied for obvious reasons.
Also, all the different ways of taking HRT have different half lives which is why you feel better or worse when low. Often doctors prescribe people valerate for injections but put people on on a 7 day injection cycle when the half life is 4.5 days, and don't give people a high enough dose for them to have comfortable levels on day 6 and 7. Ideally valerate is instead taken every 5 days to avoid a very large dose and to avoid the 2 days of being too low.
With gel and pills the half life is very short and so you can notice the same day if you are a few hours late, but blockers will change the experience a lot ofc.
As others have mentioned it's different for everyone!
In addition to all of us responding differently we also take HRT differently. I take enanthate injections every week and so at the end of the week I don't feel much of a low because my levels are very stable. Would I feel a relief if I took HRT orally or transdermally and therefore had more swingy levels? Maybe. Maybe not. I certainly remember noticing changes more easily back when I did gel, but it was also early on in my transition and so perhaps things have changed now.