What Causes Bloating, Nausea, Cramps, Headache, Fatigue And Back Ache?

hi, i had my first baby a year ago and have been breastfeeding. she only has a little bit now, (just before bed) and my periods have not returned which my doctor assures me isnt uncommon. However the past few days i've been bloated, have had nausea, lower tummy cramps, headaches, fatigue, and very low back ache. i did a home pregnancy tst a couple days ago but it came back negative. i was wondering if anyone could help tell me whats wrong?