15 days late 3 negative tests

I'm 15 days late today. I didn't have a period at all for July. My last period was june 21-25. My husband and i have been trying since April. I've taken 3 home pregnancy test and all came back negative. I'm never late on my period. i have sore nipples to the touch and a lot of clear discharge. It feels like I'm starting my period that's how much it is. I have had a little nausea not throwing up. I tested again this morning and it came back negative. Any advice? Does it sounds like I'm pregnant? Why do I keep getting negative test?/!?! I'm getting very sad about it.