Brown fishy discharge

This is the first time this has ever happened to me. About 2 weeks ago I began having yellow discharge that smelled a little off and now I am having light brown discharge that has a fishy odor. I was multiple times a day so this shouldn't be he problem. When my bf and I have sex it's without s condom and he cums inside of me. What do you think is happening? 
PS I took a std test at my gynecologist office and I am negative for chlamidya and gonnorhea.