10 month female might be pregnant?

My 10 month old female siamese cat had her heat last month on the 17th. She was meowing a lot, sticking her bum up, showing flirty behaviour, the works. This month we've noticed her stomach seems to have tensed up and her ribcage seems to be expanding (as if her womb was too) her nipples are also darker and bigger. She got out during her heat for about 10-15 minutes and we're not sure if she's pregnant or not. The last few days she's been following me around, purring very loudly, and meowing at me if im not in bed. (She sleeps on my bed every night) does anyone know what she might want/need? She seems to desperately want my attention.

10-15 minutes is long enough, if she met up with a tomcat. So it really all comes down to the tomcat population density in your neighborhood. If she's not actually pregnant, it could be a false pregnancy.

Did she ever get outside when not in standing heat? Cats don't need to actively be in heat to get pregnant---the act of mating is what stimulates ovulation so any time a tom and an unspayed female get together there's a chance of pregnancy.

It would be best to have her spayed. If you don't want kittens, now would still be a safe time to spay/abort. If you don't want to abort, you can have the vet check to see if she's pregnant---if not, she can be spayed, if so, you can make sure she gets the proper prenatal care.