What Causes Stoppage In Menstruation?

hi, i have the contraceptive implant and havent had periods for the last 4 months, however at college the other day i sat on a chair which gave way and i hit the floor hard, i felt ok at the time, but when i got home later that evening i had all shooting pains up my legs and my hips were hurting which i put down to the fall, when i woke up the next day i had came on my period........... which was a shock becayse as i say, whilst i have had the implant ive had no monthly cycle, could the fall of brought on a period?? thanks