Hi OP.
I was about to suggest PMS and then saw that another poster already had.
That's not to make light of it. It is fucking hell for some women. There is a condition called PMDD which is basically PMS on steroids and is completely debilitating.
I never suffered from any pre menstrual issues until after I gave birth to DC2, the whole experience was incredibly traumatic and I suspect that tipped the balance somewhat.
Out of nowhere came these overwhelmingly unbearable emotions/feelings bang on cue, every month without fail.
I felt completely out of sorts, not myself, cripplingly hopeless and chronically depressed, angry at myself, angry at everybody else. Everything was negative and I couldn't see a way out.
See if you feel any different a few days after your period starts. If so, its likely PMS (or PMS is adding to it)
There are things you can take, it doesn't have to be BC. I'm on a supplement regime that really does help.
Vitamin B6
Zinc
Magnesium
Calcium
Evening primrose oil (star flower oil also highly recommended)
Other things; agnus castus, St John's wort etc.
PMS aside, you've been single handedly raising a baby on your own for all of these months - that's overwhelming! If you've been on auto pilot all of this time it's only natural things would come to a head if you've been bottling everything up.
You're not a bad mum.
Your mum will understand.
You're doing great.