Confusingly, seems it can be either oestrogen or progesterone to blame (or an imbalance between the two), but if you have swelling as well as tenderness, I'd say more likely high oestrogen, especially as like me you're hypothyroid & we have a tendency to fluid retention, which can be exacerbated by exogenous oestrogen.
I was never sure what was causing my breast tenderness in fertile years as some months it was cyclical i.e. mostly in premenstrual phase, some months it persisted throughout the entire cycle & for consecutive cycles. As a long term breast clinic patient for fibrocystic breasts, needing scanning, aspirating etc, prescription Evening Primrose oil helped to some extent. When it was discontinued on the NHS I bought it privately then changed to Starflower oil, but later found fish oils (& regular rowing machine sessions) gave me the longest pain-free stretch I'd had. Swimming is said to be helpful & I did that regularly for years but tbh it didn't seem to help.
In the few years postmenopause before starting HRT, my breasts were blissfully pain-free for the first time in 35 years. Starting HRT was when I discovered that high/imbalanced oestrogen seemed to be the culprit in my case - my menopause specialist added Sandrena to my combi-conti HRT (Evorel Conti) to try to get my oestrogen levels up & the pain & swelling increased markedly, to a degree I found unnerving & which was too much to tolerate.
Maybe have a look at the following links & extract from the first. The 2nd link isn't that helpful, simply confirming the validity of the differing experience among those of us who've replied on this thread - i.e. it can be due to either prog or oestrogen!
https://www.thewomens.org.au/health-information/breast-health/breast-soreness"The most common hormonal breast soreness comes from an increase in the level of oestrogen before a period. This causes milk ducts and glands to swell, trapping fluid in the breasts. Many women have tenderness or pain in the lead-up to a period, and sometimes right through it."
https://www.nhs.uk/conditions/hormone-replacement-therapy-hrt/side-effects/I hope you can get it sorted - the pain can be considerable.
Wx
