sounds to me like you have a weak lower back and it's your weakest link and it's giveing out on you..... i would sudgest a workout you can do 3 days a week and will strengthen up you lower back pretty fast .... stand with your feet apart a fairly wide stance ... bend down touch the floor only using your lower back to bend keep your legs straight and try to keep goo posuture..... touch the floor stand back up then reach down touch your left foot ... back up then touch center again.... then reah down touch right foot... then center .... just keep repeating that process for 25 reps to start with .... aim for 3-4 sets of this each workout ... it's a great workout for people with weak lower backs ... even for guys who are in great shape it's a good workout just add more reps dependig on your level of fitness .. hope that helps ... i could be wrong it could be as simple as the way your landing on your feet while running or the shoe's your wearing ...