2nd period in one month.

tmi alert

OK so I'm 23 years old and I just finished my period sunday 17th and I got another period monday the 25th its seems to be ending again today but idk what caused it. i t wasn't like my regular period. I had no cramps or backpain. I use a menstural cup and it may be had a tsp a day and it was a dark rust red but I had alot of cervical mucus. any ideas what caused this?

PS as its ending my back is starting to hurt and I have nausea all the time but its been worse these last few days. I thought I was pregnant but the test was negative.