My 5 year old bitch who had one litter 17 months ago is now having some sticky clear fluid from the nipples, only when expressed, and I found it when she had mud caked on and I had to scratch it off.

I had a slide for cytology and there was nothing out of the ordinary, but its concerning me. I wonder if I should do an ultrasound to ensure its not breast cancer or anything.

She's due in season in about a month, and I was planning on breeding her.

Any ideas?

I have had this happen before, with a bitch in a false pregnancy. Her mother had whelped a litter, and her daughter went crazy hormonal.
Were there any other bitches pregnant or whelping in the household? That would be my guess, that she is in a "sympathetic" false pregnancy. Could you have missed a subtle heat cycle two months prior?
I would not be overly concerned, since it's more than one nipple, so it sounds like a hormonal thing, like false pregnancy. Don't worry about cancer, especially if the cytology was normal.

From a human perspective, I have seen that personally.

My 12 y/o bitch has had this since her final litter 8 years ago. At that time I was so panicked b/c she had a clear or sometimes blood tinged discharge from her 2 rear nipples. We had one of the breasts removed on advice from vet. It was clear. As I said, the remaining nipples still expresses this liquid.

My bitches have this every heat cycle...it is the cholostrum (sp?) that is part of a false pregnancy. The person who also saw blood probably had a bitch with a touch of mastitis...masectomy is sure a radical treatment for that. most people just don't even notice their bitches go through the clear sticky discharge from the nipplesbecause unless you investigate ddown there...it would be barely noticeable. It will dry up on its own. Don't express her nipples too much or she will begin to produce actual milk....she is just an excellent mother. She will by absolutely fine. Good luck with her upcoming breeding.