My hamster has a lump and is struggling to walk. What should I do?

Thank you for using Petco Pet Education Center, formerly Petcoach and congratulations on your new additions! I can see a darker area on Rosie's bottom, but it is difficult to determine if the lump is actually protruding. Hamsters do have pretty prominent scent glands, and this may be due to poor grooming of the secretions. Older hamsters have a tendency to develop papillomas, which may be uncomfortable and can cause the trouble walking. You should have Rosie seen by a vet to determine exactly what the growth is, they may take a small sample with a fine needle to send off as a biopsy. In the meantime, keep a close eye on the area for increase in size or bleeding, as these are indications for an emergency visit. I hope all is well!