Overtraining?

How can you tell if you're overtraining? I'd like some "expert" advice from some 50 somethings (or a coach of 50 somethings!) about how to know when what I'm doing is too much. I've added weights to my swimming - I go three times a week for weights - which I follow by a fairly easy swim to loosen up my muscles. I'm still frequently sore the next day (and the next, and the next) however - but I know this is to be expected - if I didn't feel sore, I wouldn't feel like I was actually even pushing myself. the other three days of the week, I generally put in a good 90-120 minutes workout, getting in from 4500-6000m. Sometimes, like today, I get to the point where I just can NOT muster up the energy to put any real effort into the swimming - today, for instance, I did fairly well for the first 3000m, but then once we started the "real" workout, the first main set, I was just pooped. I could swim, but at only a slow pace. (By this I mean that an interval I usually can keep on a 200 by about 10 seconds, I missed by 1 second - and I was wearing zoomers. And the thought of doing like a 50 sprint, was out of the question.) This was NOT a day that I swam following weights, BTW. Do you think this is a nutrition problem, and that I'm just not getting enough protein in my diet? I have to say categorically, that eating BEFORE practice is out of the question - I swim at 5:30 in the morning, and would probably barf if I ate before practice, although I do sometimes eat some Shot Blox. Am I overtraining & need to cut back somewhere? Or is this a short term transition (it's been going on & off for weeks now) & I need to just hang in there? Thanks in advance!