I feel your pain, because I have seborrheic dermatitis too. Sometimes, I feel so hopeless, because nothing seems to help.
I think antibiotics could definitely be the cause of your flare up. I've read that one of the suspected causes of seborrhea is yeast. Maybe the antibiotics are upsetting the balance of bacteria that was keeping the yeast in check.
My sister also has the condition. After being prescribed just about every medicated shampoo on the market, she found a doctor who put her on antifungal medication. Her condition cleared up quite a bit.
Treatment of Seborrheic Dermatitis - May 1, 2000 - American Family Physician
Hopefully, your scalp will recover once you're done with the course of antibiotics.