I didn't feel like this when I was pregnant with my first and I'm not sure what it could be

I had my first baby in Oct and since have been on BC. My husband and I had sex this month during the most fertile time in my cycle. My period had started thursday and ended sunday. It started off with really bad cramps and bright red then tapered off the next day to the same but really light and saturday it was nearly gone except for when I wiped. My breasts have been sore and I'm far more tired than I ever have been. Some smells make me really nauseous, especially when I changed my DS's diapers. I've had minor cramps since my period ended and my breasts are not as sore or tender but my lower back has been killing me and I've not been able to have a regular BM for the last couple of days. I didn't feel like this when I was pregnant with my first and I'm not sure what it could be. Any advice? thanks ladies.