I Was Told That Someone I Recently Had Sex With

I was told that someone I recently had sex with I hiv+ ,,, I asked them and they said there hiv- but there ex girlfriend who they have had unprotected sex with is hiv+ that they are taking a drug called prep so they don't get hiv since I have had sex with this person should I get tested as well ? I took them to Walgreens to take a test but I'm still concerned... It came back negative but I was reading if they were taking the medication for it that it could be a false negative