She was spayed a few months ago and since then started peeing inside which she has never done before. This week she even peed inside during the day - once right in front of me. Why has this suddenly started? We let her out often so there is no reason for it. Is there something I can spray the carpet with to deter her?

Is she actually squatting down to pee, or does it look like she has no control over it?
There is something called spay incontinence, where after the spaying the muscles around the bladder just aren't strong enough for them to be able to hold in their urine.
With this form of incontinence though, they usually are constantly leaking urine.

She may also have a bladder infection, making her need to urinate frequently.

If infection and incontinence have been ruled out then unfortunately you may just have to go through the potty training process with her again.
Some dogs seem to 'forget' where they are supposed to go after they have been away from home for any period.