first of all: her 'heat' i.e. is the blood, is only the BEGINNING of the heat : count 9 to 12 days after the start of the heat and that is when they can be bred. if she wasnt 'tied' (that is, stuck to her, and twisted around, so they were bum to bum) on those days than most likely she's not preggnant but THEN , pymetra is a dfeinate follow up to being in heat and not bred... and that is a life threatening disease. bitches die from pymetra. ive taken out many a pus filled womb from a dog right before it ruptures which kills them...
when in doubt, have the vet check her. no need to spay if u want the pups, but if its a pymotera, then they will do an emergency spay unless u want to try to save her uterus for future breeding, but frankly it doesnt seem thats what u waould ant to do...
pymetras are sneaky since they are contained in the womb the body of the animal doesn often show signs other then as if they are pregnant, and or drinking water heavily, not eating much, getting fatter, and sometime, if u are lucky, vaginal secretions (greenish), so the infection is draining...
most bitches dont show signs of pregnancy until they are very close to giving birth, we had a canaan dog that surprised us entirely with her litter.... some dogs, like people, 'dont' show' the first time...
good luck