I tore my ACL a few years back and had 2 operations keyhole and reconstructive since then ive played football?

alot and it has given way now and again but the doctor said that it was just stretched muscle and i had nothing to worrie about, Recently i fell on my knee whilst playing football and ever since then when i do any physical activity its like something pops out of place when im bending my knee and i have to straigthen my knee for it to pop back in place very painful..does anyone know what this could be?