Please help!!! Please!

I was 4 days late on my period, never usually late, so I thought this was it, thus morning ad arrived and I felt fine, about 12pm I went really light headed & faint... I went to the loo & I had severe intense period like pains so intense it bought me to tears and I couldn't move from the bathroom floor, I then felt extremely nauseous & vomited! I went back up to bed & laid there for a bit, 30 mins later, sickness went, cramps went and now my bleedings extremely light & no cramps at all! I never usually have severe or painful periods, it was sooo excruciating ! What happened in that 30 minutes?! Xxxx