Attempting to mount another is not just for breeding, but also for dominance in play, or even the odor of an anal gland in excitement that gives the appearance to the other dog of being in heat. Especially being pregnant when the digestion system slows down, it is possible she could have a mild anal gland infection. I would check her by the vet. Could be as simple as they express it, or more so with antibiotics, but it would be better to go earlier before the puppies are born as they are not nursing yet.