Difficult to answer without more information, but a pregnant guppy should be noticeable, they do get quite large when pregnamnt. They can give birth to nearly 50 at once.
A pregnant guppy is very common, but guppies do get stressed if they are put with other fish and then isolated and then put with other fish. I highly suggest you isolate her right
Yes. Really, without more information than that, there's no way someone on the internet can tell you. Is she getting large? Are there little fish showing up in the tank?
