I had a crown put on a cracked tooth 3 months ago and it was fine but now it has started aching?

Needs reevaluation. Could be fracture is worsening with resultant death of nerve of tooth. Depending on severity may require root canal treatment or removal and replacement with implant/crown. May also be that the new restoration is ever so slightly too high and a simple occlusal adjustment will eliminate symptoms. In any case, return to your dentist for evaluation and rx.
See your dentist. I would go back to the dentist who placed the crown and ask questions. It could be your bite or possibly the nerve in the tooth is gradually dying and you may need a root canal.
See details. Have your dentist evaluate the issue to be sure there is no underlying decay or abscess.