Nausea, vomiting first day(s) of period

For the past 10 months, I have been experiencing severe nausea and vomiting on the first day of my period. I have an insanely irregular cycle, but 2 - 4 hours after the bleeding starts, so does the nausea and then vomiting.
I've talked to my GP about it, and he's given me Pramin to treat the nausea and says to see if it comes back, even though it's happened 8 times in the last 10 months. The Pramin barely works, and he's now said to take Stemetil instead.
I had a laparoscopy 2 years ago to see if I have PCOS or endometriosis, but apparently I don't have either.
This is driving me nuts!
Does anyone have any suggestions as to what could be causing this and what I could do to at least survive the nausea and vomiting?