IBS, Antibiotics, or a Bug?

Hello all! I just recently--2 days ago-- started having diarrhea. I have been on antibiotics for a UTI for exactly 1 week today. I am also taking a pro/prebiotic. My diarrhea has worsened texture-wise and this is all foreign to me. Since I was a kid, I've been generally constipated. There is no blood, external hemorrhoids or anything out of the ordinary. Just diarrhea out of the blue.

Should I worry?

For those who are medical professionals or just curious, I also have mild GERD, but I keep it under control with Omeprazole, Ranitidine, and Gaviscon as needed.

Also: please don't just say to go to the doctor. That's not really an option for me unless it's serious.