Maireas She was certainly abused - inappropriate touching - by Somerset, as a teenager. And there were certainly rumours about her and Leicester and Essex and others. She may well have had emotional and - to a certain degree physical - affairs (so far as we know, she almost certainly did) , but I think she was far, far too canny to have risked pregnancy out of wedlock.
Politically, she made great and very effective use of her claims to virginity for political purposes. And on her deathbed, she declared that she was a virgin. This was at a time when people genuinely believed that sinners were puished in hell. So far as I - and others - can tell, she was not personally devout, and was very, very pragmatic when it came to religion. But telling an open lie on your deathbed would have been an absolutely enormous and shattering thing to do for anyone at that time.
We don't really know much about her gynaecology. There were hostile rumours that she was physically not capable of sex; on the other hand, spies for one potential foreign husband quizzed her washerwoman (!!) who reported that things were normal - though of course we don't know what precisely the washerwoman meant by that.
You are correct, in that she might have had periods for a long while. But she was extraordinary - a complete and utter exception.
Having said all that, not all women married in Tudor times, and, as I said above, poor ordinary women did not usually marry until their mid 20s. However, there is some evidence to say that poor nourishment and tough physical work (in the fields etc) meant - compared with today -that periods started later, around mid- late teens. And might sometimes have been delayed/non-existent.
And, of course, the Church taught that periods were part of Eve's punishment; they were expected to be troublesome. If you'd been brought up with this outlook - and had no other possible explanations presented to you - then you'd expect pain, discomfort etc etc etc.