[lo lo 1981]

Last month I had a period on the 7th April and then again on the 30th April (23 days apart) my cycle is usually regular @ 28-29 days this has never happened to me before. The second cycle started extremely light but became heavier by day 2 but still lighter than usual ( 1 pad a day would do) and was bright red in colour and quite watery. It lasted 3 days and then totally stopped at which point I began getting stomach cramps and headaches. In the last 2 weeks I have been feeling bloated and have a strange fluttery sensation in my abdomen and a rash has appeared on my stomach (itchy at times)... I stopped taking birth control after just over a year in Feb 2012 and have had regular bleeds since without cramping. I have a 2 year old child and had an ectopic resulting in both my tube and ovary being removed almost 5 years ago. We are Trying to conceive at the moment too. How do I calculate the date my next period should arrive will I calculate from the first date I menstruated in April or the second date in which I menstruated in April?.
I also took a HPT on Monday which was Negative, could the second bleed have been Decidual bleeding.

Please advise me as I am confused.