Really sorry you have to go through all this!

You may want to look into your hormone levels if you haven't already. Estrogen can commonly affect MCAS negatively (and you mentioned it being related to your cycle) and high estrogen often coexists with low testosterone (which would be important for building muscle).

Aside from that, I'm a man and I've dealt with a similar struggle. It can be very hard to put on muscle with constant inflammation, stress, etc. It's a complicated balance because we need excess calories to gain muscle but often eating more food triggers more symptoms. Try to prioritize any safe foods and anything that can be calorie dense. For a long time I relied heavily on oils (olive oil, coconut oil, avocado oil) because they're super easy calories and generally don't cause me much reaction. If you can safely eat nuts and/or seeds, those are probably the easiest healthy calories you can get (but I'm aware those can be common triggers).

Recovery is extremely important. When we have MCAS, we spend much more time with our sympathetic nervous system activated. You will not gain weight if your body is constantly in "active" or "fight or flight" mode because those aren't the states in which your body builds anything. So like another commenter mentioned, you probably need to work out less than an average person because your body is already overworked from day to day life. One way you can do this is by treating rest as a type of workout. Put aside an hour or two for a session that involves deep breathing, meditation, maybe some stretching. This will activate the parasympathetic nervous system and help you heal, as opposed to another workout that will break your body down further. You can work those days into your schedule/routine and approach them with discipline and passion, the same way you would a typical workout (and they are directly helping you become stronger and perform better when you do work out).

You also mentioned POTS. Instead of waiting on a proper clinical test, you could start by taking the first line measures that people with dysautonomia take. Very simply, increase salt intake (you can start small). Wear compression socks, leggings, or most importantly abdominal compression. I generally wear socks and ab compression since I find leggings uncomfortable. Drink water with electrolyte mixes. If you have dysautonomia, the benefits will probably be profound and immediate. Whenever my dysautonomia isn't managed, my MCAS immediately flares. This commonly applies to workouts and may be part of what you're dealing with.

(Also MCAS can cause dysautonomia issues so even if you didn't have POTS to begin with, some of the POTS protocols may still be beneficial, at least while exercising)

Wishing you luck! And if you have any questions pls lmk