My chihuahua has what looks like little lumps of dried blood on her stomach, along her cesarean scar (which occurred 2 years ago and before she was mine). The blood is not easily noticible and I have to look really closely amongst her fur. It also occurs around her genitals. I can not see any wound that it could be coming from and she is behaving normally other than when she was lying in her bed she suddenly gave a yelp, jumped up and ran out the room! We are going to the vet but in the meantime I can't stop wondering what it is and worrying. At first I thought it might be flea droppings but I've triple checked her and can't find any actual fleas. Any advice or suggestions would be much appreciated.

If it's dried blood, then a tiny amount placed on a damp tissue will turn the tissue reddish brown.  Flea dirts tend to be crunchy........

It could be that the fur is just stained reddish brown, which can happen when it has been licked a lot, it doesn't have to have been recently. I have a bitch like this, she has the red colour around her spay scar, and around her vulva as well.

You can't always see fleas but you certainly see their dirt.  Flea dirt consists of dried blood.  Sudden yelping can be a sign of illness, discomfort so a trip to the vets would be advisable to rule out any medical condition - however, I've known dogs that get bit on the bum by a flea and they run around the house like a loon before stopping in mid run to nibble away at the 'sore' spot.