Why might a teenage girl who is using birth control be more at risk for becoming pregnant than a woman in her thirties?

The main reason would be that some forms of birth control may be less effective for teenage girls because their bodies haven't fully matured and settled into a regular menstrual cycle. In addition teenage girls are more fertile than women in their thirties and have a higher chance of conceiving even while taking birth control.

avoiding recreational drugs instead of using them

these are common warning signs of overtraining syndrome:

washed-out feeling, tired, drained, lack of energy

mild leg soreness, general aches, and pains

pain in muscles and joints

sudden drop in performance

insomnia

headaches

decreased immunity (increased number of colds, and sore throats)

decrease in training capacity/intensity

moodiness and irritability

depression

loss of enthusiasm for the sport

decreased appetite

increased incidence of injuries.

a compulsive need to exercise