[My baby*Fergie*]

Well I know I typed a large thread before with the problems I had with Fergie, well now I have a couple more. She had a check up when she was fixed ...and they were saying everything is fine with her... well what Ive noticed is her butt REALLY stinks....but it doesnt stink like poop.....it is really not a nice smell at all...i noticed when she was sitting on my lap.....its been like that for a few days......I have no clue what it is....on top of that she has a few red marks that have appeared on her skin around her stomach area....the only thing I can think of is that we switched her to Nutro Natural choice...now all this stuff is going on...her eyes tear more than ever...and her coat is more of a mess. Should I switch her to Royal Canine ??? some advice please!!!!