pregnancy risk? pls help an anxious guy.

hi, this is my first post here so im kinda nervous.

i had sex with my gf 3 weeks ago (18/08/2017) while using condoms and being very careful about everything (she's not on birth control pill etc). last week she had PMS symptoms and said that it was ok.

but the thing is: i cant get out of my mind that she might be pregnant. her cycle is 28 days so on the day that we had sex she might have been on the 'fertile window'. And today (04/09/2017) she's expecting her period to come (didn't come yet).

i know that the chances are REALLY low, but im a very anxious guy (srsly trust me)

we had sex with condom, i checked for any holes by putting water on the condom and nothing came out. the other time i ejaculated on her breast and cleaned up after a minute or two. she's not nervous as me btw, shes calm etc.

please help a anxious guy.