Missing something in My Diet?

I did Search the forums for other posts on the Subject of Diet and found some results but they tend to cover a very broad area on Diets in general. I may not be able to get a quick, specific answer I'm looking for (maybe there's just too many variables) but here goes. little background: I'm 50, Male, Breaststroker, but took some time off and just swam for light fitness and put on a little weight. Never been a "nutrition aficionado" and started training again about 6 mo's ago and I've been on a diet for about two months -I've lost 20lbs which I'm very happy with - 6' 190lbs.. I feel great overall and the Diet has done almost everything I wanted, however I've noticed that I just don't have as much muscle-power in my arms that I had prior to the diet. I eat a lot of fruits and vegs - 3 meals and a snack a day and a lots of water. The Daily animal-protein is at dinner which is either Chicken, Fish or Lamb. To put it in the form of a question, if everything else feels good (energy, aerobic, etc) and but my arms (biceps and pecs) feel weak, what could I be lacking? (more animal protein or a food group etc) I've been doing some dryland to see if that offsets it, but "my gut feeling" tells me it's something lacking in my diet after the weight loss. As i said it may not be an easy answer without providing more info, but wanted to give it a shot