The problem with the Middle Ages in England is that there is much less writing in English than there is in French or Latin. However, we do have the Canterbury Tales which is a fairly extensive corpus for Middle English words and also has the benefit of being written in "vernacular" English, as Chaucer says himself (at lines 727-28 of the General Prologue):

“I pleynly speke in this mateere / To telle yow hir wordes and hir cheere”

Admittedly the Tales are from the turn of the 15th century, so later than your question, but we see a lot of words that are related to our modern swear words and a fascination with toilet humour and genitals:

[Absolon was] Of farting, and of speche daungerous (Miller's Tale, l. 235)

And at the window out she putte hir hole

And Absolon, him fil no bet ne wers,

But with his mouth he kiste hir naked ers - (Miller's Tale, l. 628-32)

'Ers' is what we now say as 'arse/ass'.

We also see the ancestor of the word 'cunt':

Pryvely he caught her by the queynte (Miller's Tale, l.175)

And furthermore:

To see a shitten shepherd and clene sheep (General Prologue, l. 506)

This doesn't tell us whether these words were offensive and used as swear words at the time, but it makes it clear that genitals, scatological humour and so on were sources of humour and quite tittilating for your Middle Age reader. Chaucer's characters are purposefully vulgar and they use these kinds of words.