Sudden change in menstrual cycle

Hi, I had a very sudden change in my menstrual cycle last weekend where I experienced SEVERE pain (i mean genuinely crippling), sweating, shivers, nausea, and dizziness. Thankfully after a couple hours and 800mg of ibuprofen, the pain reduced down to what I would describe as my normal period pains.

My period was then very light and brown in colour day 1, light bleeding day 2, and very light bleeding/ basically no bleeding for day 3&4.

I have contacted my GP and have been booked in for a smear test. This will be my second one. 

In the meantime, I was wondering if anyone else has ever experienced anything similar?

I have had some breakthrough bleeding while working out before which would also be abnormal for me. I've had my period since I was 8/9 and am now 28 years old. Usually it is quite heavy and has quite large clots, but is always regular and lasts around 5-7 days, with what I would describe as bearable pain on day 1 and very light cramps for the remainder.