Dogs can go into heat as early as four months old, so she very well could be. I would have her spayed. My vet won't spay until 6 months but many do over 4 months and my shelter does any puppy/dog over 7 weeks old and kittens over two pounds.
Roxy that isn't an absolute sign your dog is going into heat. One dog with mount another for different reasons. There isn't an exact time that a female will go into heat either. Could be six months or much later.