Nope, you can't get pregnant from grinding and especially with clothes on. While pre ejaculate does have sperm in it, it wouldn't get in you in any of the ways you mention. Sperm is smart and all but it does not go in and around clothing or even swim from 'near' the vagina into the vagina. Sperm will not travel through clothes. No reason to worry about pregnancy with this activity. Periods can be late for a variety of reasons including changes in routine, illness, stress, weight gain or loss, etc. good luck