My 3 month old cat usually goes in her litter box, however she has started using my blankets and bed instead. I never seem to catch her in the act, so how can I stop her from doing this?

Most cats who inexplicably start with this sort of behavior are trying to tell you something - usually its something along the lines of "I'm sick" or "I'm in pain". With my boyfriend's cat, she started doing this and when brought to the vet, we found that she had a terrible ear infection. I'd suggest taking your cat to the Vet asap.

First keep her out of the bedroom, also provide another 2nd litter tray, maybe a different type of litter? also try newspaper shredded (messy) but sometimes helps.
She is marking you as hers ! or it could be a stress thing, I do not have enough information! however it is hard to stop once started. YOU MUST keep her out of your bedroom, even get a decent sized kitten pen for a few weeks, also it is hard to get the smell out, and you may even have to buy new duvet/sheets/pillows/cases and possibly mattress - they can smell it forever.
Another idea it to tape black dustbin bags together to well cover your bed, weigh the edges top and bottom down so she cannot get in it- she will not like it when it bounces back! but get it all really cleaned out first and get Pro Pack deodoriser - found this to be the best. Good Luck hon!
Sylvia Hauser www.sharue-siamese.co.uk

A couple of possibibilities that may help you.
One- cats tend to urinate on beds and blankets if they are made of down. If you have any down (like goose) items on your bed, either pillow, blanket, pillow top beds they will smell it and try to mask it.
Two- If you have more than one cat and the other has privilages to the bed or is not allowed on by it the cat with the problem will mark it's territory when it gets on to the bed.