Yes you had sex at your most fertile time. Yes the chances of concieving are high. If it is your first it normally takes around 4-6months to start showing .. if it is your second or so on you can show as early as 2months as your muscles are not as tight. Your body only makes a baby big enough for you to carry so you will not have some massive baby just because you are small. I did not start showing until I was 6months pregnant as I had tight stomach muscles prior. You will not start showing sooner than the average woman as the baby starts off very small ... smaller than your finger nail. Are you trying to fall pregnant. It sounds like you are. Go get a pregnancy test and if you are not trying to have a child use contraception!!

The first sign of pregnancy is when you stop getting your period. When that happens, you should take a home pregnancy test or see a health care provider.

Showing takes months.

I didn't see the part about what prescription drugs you are on. I know it is bad to stop your meds straight up, however they could harm the baby if in fact you are pregnant. So I'd advise seeing your doctor TODAY to check if you are pregnant so you can work out with your doctor if you should continue with your medication.