Why is my guppie bullying my pregnat guppy?

i have a pregnat guppy that is about to give birth in about a couple days but one of my other guppies is bullying her alot... picking on her/trying to bite her/dosnt like her... shes also doing it to another guppy thats pregnat. i know this is a sign of pregnacy but why do they do this. how can i stop it. i dont have another seperate tank and i dont have another 70 bux to spend on another tank. ik they need heat and filter and everythig and i cant spend another 70 bux for all that to set up anoth 10 gallon tank. i cud spend a good 10/15 bux but i heard they need at least a 5 gallon. what should i do?