Itching and watery pimples on the buttocks can be caused by viral infections (like herpes or shingles), contact dermatitis, or bacterial folliculitis. Because treatments vary, you should consult a doctor for a proper diagnosis. If it is viral, you may need prescription antiviral cream (like acyclovir). For allergic reactions, an over-the-counter 1% hydrocortisone cream or calamine lotion can help soothe itching. Meanwhile, keep the area clean and dry, wear loose cotton underwear, and avoid popping the blisters to prevent a secondary bacterial infection. Please see a healthcare provider for an accurate prescription.