I'm having post pulpectomy pain! help?

It will get getter: Sadly sometimes you develop pain secondary to instrumentation of the canals - apical periodontitis. Depending on what medications you were prescribed and what you might be allergic to - taking an anti inflammatory med like advil (ibuprofen) / Ibuprofen 400 mg every 4 hours can help. If any doubt please contact your dentist first.

Tooth pain: Two possible reasons. The first being that not all of the nerve was taken out. The second reason is that the pain is coming from the nerves outside the tooth. A pulpotomy can't get rid of the nerves that surround the tooth. It only removes the main nerve that's in the root canal.