I have a pain in my knee caps and I have it after my period all the time, I also get these sharp pains from my bum all the way to my waist help me?

Pain in knees. This could be what is called chrondromalacia patella. This is an "unevenness" behind the kneecap to describe it in layman's terms. You may also notice after sitting for prolonged periods. I would strengthen the quadricep muscles to help with this. The pain in your bum, not sure - either from back itself or from the rectal area due to spasm.
JHS. Best explanation for anterior knee pain & sciatica associated with menses is unstable pelvis w. greater ligament laity from Relaxin secreted w. menses; causing sacroiliac joint subluxation. SIJ hypermobility incites sciatic nerve irritation. Hypermobile females oft have genu valgus, causing patellofemoral syndrome, which is exacerbated by pelvic girdle wobbliness causing rockiness of knee below. .