@ehBeth,
At 5ft 8in and at 135 lbs, it sounds as though you might be 20 lbs underweight. At age 17, that's not too unusual. Sometimes there are growth spurts even at age 17 and maturation can take care of that. Weight training is good but it's limited. Cross-training into other fitness activities keeps a person from getting stale like that. Over-training can drop your performance markedly.
I experienced that sort of regression as an adult runner and as a remedy, took 3-4 days off. I bounced right back within 1 week.
Pardon me here as I don't mean to speak for USMC, so I won't - but -
I think we're going to read that he
didn't adjust his over-training routine. The problem with over-training and being 'under-weight' is that the body breaks down -- first by making you feel tired a lot and then, after that point, the max amount you comfortably lift drops.
Soon after that point, other injuries may follow. Resting for a few days is advised,. Then dropping to lower weight for curling and then doing more reps can get you past that.
Also, you state something that has been missed. Your diet and your weight loss of 5 lbs is pretty telltale. You shouldn't be losing ANY weight especially.
Are you doing cardio too If you are ...drink more water too.
What you write is showing you need a higher caloric intake. Perhaps 25% higher. Add those proteins shakes. But drink a lot of water throughout the day. Higher protein intake means you need to flush the kidneys with more water.