My boyfriend only pre cum on me. I took an after pill before 72 hr (at 43hr). What are the probabilities that I can pregnant?

Present but low. Any ejaculatory fluid reaching vagina has potential for causing pregnancy. Pre-cum can certainly contain sperm ; escapes before male reaches climax ; hence need for condom protection, which in itself is also.
Agree w Dr. Klauber. Use of emergency contraception really reduces risk. There is potential for pregnancy if sperm is deposited at the introitus (opening of vagina) or within the vagina. If not ready for pregnancy – use protection every time.