Period & Pregnancy bleeding confusion.

Hello, This is happened 2 month ago. Me & my gf is doing dry hump sex means we both are in underwear and just intimate to each other and she wear nylon underwear and I wear cotton underwear so when I releasing my sperm her underwear is little wet due to Sperm but my penis is never entered on her vagina and both r in underwear. Then we worried about pregency and waiting for her period. 1st month (17th March) period is normal but Quantity is little less as compare to other times . 2nd month ( 15th April ) period is also happened but same quantity is less and on a 1st day some strings type Sperm is releasing and colour is also different. Is there any chance of pregnancy??? How to differentiate Normal period and pregnancy bleeding ???