Hi Littleacre
Any warm swelling is usually a sign of infection, maybe she's picked up a bug via a nick or cut on her stomach or teats. I would take her temperature and if its above the norm 38.8 deg C administer a long lasting antibiotic. If she's off her food too this is also a sign all is not well. Sunburn wouldn't necessarily affect her undercariage unless she's been laid flat out in the sun for a long time, you could try mopping the affected area with tepid antiseptic water too. Also try and get some liquid into her, diluted fruit juice, milk or plain clean water, we use a 500ml water bottle to do this.
Would have a chat with your vet, chances are he/she will make the same assumption as me and administer AB's. HTH
Hope she recovers soon, all the best
mandy