I have been on low dose birth control. is there a chance of pregnancy if i take monodox (doxycycline monohydrate) 100mg for acne 2 nights after having sex?

Low risk: The risk is low but there are options. You could check to a more effective birth control method such as an iud or change to a different acne medication such as topical creams.

Yes there a chance!: Yes there is a chance. Antibiotics can decrease birth control pills effectiveness also low dose birth control pills may not have the same level of effectiveness as older higher dose . Using a condom will improve you protection.