I've had this happen. I've also heard of it happening quite a bit, actually - so don't feel alone!
In my case it had to do with low T (got some blood work done), even after 3 days post IM when it should be peaking. That being said, low T isn't necessarily the cause - the endometrium can and does still thicken on T since endometrial cell proliferation (& tissue growth) is regulated by estradiol then progesterone chronologically in a cycle. It also may not be a full on period like pre-T, since it all has to do with the frequency and amount of tissue growth, which is usually reduced when on T.
I know it totally sucks about healthcare, I'm dealing with the same crap :/ Really the best way to figure it out if it's recurring is to get a good panel done with T, E2, and P levels. However in the meantime, try using a heating pad and/or some slight pressure. Also, if you're a coffee drinker or smoke, it can make cramps a bit worse from vasoconstriction. Generally speaking, vasodilators can help, and there are some homeopathic ones out there!
Long response, sorry. Try to hang in there and know that it happens, and you're not the only one!