What Causes Missed Periods,weight Gain And Negative Pregnancy Tests?

I am 41. My last period started July 22. I did not have a period in August. September 20th I had a little bit if brown/red discharge for about a day and a half, which was not enough to wear a pad. I have gained about 7 pounds in the last month. I have taken at least 5 different pregnancy tests over the past few weeks and all are negative. I have never had a child. I had sex on August 20th.

What are your thoughts? I mean a negative test is negative right?