Unprotected Sex, Pregnancy Test Negative, Have Extra Heavy Flow Cycle, Now Flow Less, Severe Cramps, Back Pain And Hot Flashes. Miscarriage?

My husband and I have been having unprotected set since April 22nd this year. I took a pregnancy test two weeks ago and it was negative in both blood work and hone test. My cycle has been coming on the 28th of the month starting in April. It didn t come on until May 29th and it was very lite flow in and off with a pink color. Not much if a flow just a strip here and there. Then it was a bigger strip of brown with pink. Yesterday it was mostly brown and then bright red again not a flow a strip here and there. I ve always had extra heavy flow cycles so this is a huge difference. I had severe cramps two days before it came and the cramps are still strong. I ve always had severe menstrual cramps but no mire than the first two days. My back hurts really bad, the lower part that is. Hot flashes really bad. I am 26 years old with three small kids. I ve never had a miscarriage could this be a miscarriage even if two weeks ago pregnancy test said negative as well as blood work?