Cats don't get periods, as such; they're induced ovulators.  (IOW - they
don't ovulate & get pregnant as a result of mating at or near the time of
ovulation; instead, mating induces their ovulation.)   If her nipples are
swollen & pink, she's likely pregnant.  The gestation for a cat is just a
little over 2 months: 63 - 64 days, IIRC.  I've only been around mother cats
after they've given birth, not immediately before - I don't know if they
"show"/spot prior to a birth, but it doesn't seem like it'd be out of the
realm of possibility.

I take it that if she's 3 & you assume she could be pregnant, that she
hasn't been spayed.  So my advice would be: if she gives birth, to get her
spayed as soon as the kittens are weaned, or if it turns out she isn't
pregnant, to get her spayed ASAP.  If $'s a factor, explain to the vet & ask
for low-cost ways of doing it.  The vet may work w/ you on this or refer you
to other places that have low-cost spay/neuter programs.

Cathy

She could have a health problem from either mating with a sick tom, or from
not being spayed.  If she's having discharge, then it's an emergency to get
her to the vet.  She could have a life threatening condition called
pyometra, which female cats that aren't bred will get.  The uterus becomes
horribly infected and can actually burst, much like an appendix, and if it
does that, then she WILL die without veterinary attention. This will NOT get
better on it's own. Take her in to the vet NOW.