important question

I'm just about 9 months post partum and since I got my iud in August my periods have been very long, 1 to 1 1/2 weeks long

I thought I got my period a week ago but it ended up just being spotting for like a few hours... I've been a little crampy and pretty nauseous for 4 days in a row at the same time everyday, which is how it was with my daughter.. I took one of those really cheap 98¢ tests but it was negative so I'm pretty clueless. does anyone have any advice???