Im I allergic to the sun? Please help

I started taking the antibiotic bactrim a while back. It says avoid the sun and that it can cause you to become sensitive to it. I stopped taking it like a month ago,maybe longer.I called a pharmacist and they said it takes 3-4 days for a medication to get out of your system. Whenever I go outside and the sun hits my skin for 5 minutes it starts burning,it doesnt turn red nor do I get bumps or a rash. It just fells like its burning but it doesnt change. It lasts for a long time,I put on spf 100 and that doesnt seem to help. It is mainly at the top of my body,like my head,face and chest (mainly). Sometimes whe I take a shower I get a red rash on my chest, randomly. I dont know if that has to do with this. Please help,I recently got diagnosed with alopecia so I dont know if I can deal with this.

Thank you, do dermatologists give this test? or a primary care? I hope it isnt anything permanent :(