3 weeks of cramps, AF is late and 3 negative HPT's

I've been on sprintec (b.c.) for the past year or so and my dr has me skipping the white pills (period pills) every 2 months so. My last period was June 6-15th. I started azithromyacin (antibiotic) on July 5th...2 pills the first day and one per day for the following 4 days. Not thinking anything about it my husband and I had unprotected sex that night. I thought about it afterwards and was told that antibiotics lessen the effectivenss of b.c.

I should have had my period on July 18th and have seen nothing. I stopped taking my b.c. pills July 17th just in case I was/am prego.

On the 18th i had some clear, thick, sticky, long discharge followed by brownish discharge but only had discharge when I wiped. I've also been having constant cramps and a backache for the past 3 weeks. And most recently heartburn after I eat and nausea off an on.

I've taken 3 hpt's and each has come up negative.

If I'm not pregnant then why the cramps? I haven't lost weight, changed anything or been under any more stress than normal so what gives? I'm planning on going in for a blood test soon to make sure.

Anyone have any ideas???