I took Plan B twice in two weeks and my cycle was shortened to 19 days. Should I be worried?

No. Plan b is designed to force your period earlier than expected. Taking it twice in two weeks is likely to really change your cycle for the next month or so. I would also recommend taking a pregnancy test in the next couple of weeks just to make sure it worked! if your irregular bleeding persists for more than one or two months, you should see your doctor.
Probably not. While emergency contraception is good to have around for the rare episode of unprotected sex, a broken condom, late withdrawal; perhaps it's time to not push your luck any further? See your doctor to talk about the many contraceptive options available. All contraception is safer than the alternative... Pregnancy.