I'm concerned about my kitten. Her anus is protruding slightly.

She is almost 4 months old and this has been going on for the past month. The vet told me not to worry about it, but she never actually saw it sticking out (it comes and goes). This is my first cat so I wanted a second opinion. She has already been  treated for fleas.

Here is a picture of what I'm talking about. Sorry if I gross anyone out, I'm just trying to help my kitty

Here it is

I have no clue what the problem is but I would ask the vet again if she was my kitten.  It may be something simple but I wouldn't take chances.

Hmmm, if that was my cat I would seek a second opinion.
Does she seem to be in any pain? Using the litterbox normally?

Call another vet or animal hospital im worried about your cat

The picture is really helpful.  Can you talk to your vet again and show them the picture?  It might be a prolapse but it needs the vet to see in case it is something else.  Prolapses can happen, particularly if your kitten has been straining when going to the toilet - or had diarrhoea.  You should show your vet, see what they think it is, and if there is something that can be done to stop it happening.  While it isn't there all the time she might find it irritating and bite or scratch at it and cause damage and pain even if the protrusion is simply a (prolapse) bit of internal tissue being pushed out.

If your vet has seen the picture and said it's nothing to worry about find a different vet, if your vet hasn't seen the picture show it to them.  If it comes and goes it could be a partial prolapse.  Those can get quite serious quickly.