14 and I think I might be pregnant..?

to cut a long story short I masturbated in a place where a man had just *** and it was a warm moist room (bathroom) so I didnt think anything of it. Now my period is late and the dates work out exactly if Im correct. What can I do??? I can NOT tell my mum. I know I should buy a test but i dont want anyone to know...!!!! I plan to just presume that I am pregnant so I can get used to the idea and plan so then if my period does come at least I would have been ready. This way if my period doesnt come, it wont be a huge shock! Im going to see if my period comes or not...ill give it another week! Im soo scared! I cant concentrate, I feel sick and Im so so so worried!