I was in a very similar situation this past summer. Getting faster on the bike... and having to tighten the belt every day it seemed! I got some good advice in an earlier thread on this page. (title: Weight getting too low?) I'd suggest checking it out completely, but here is the some of the most helpful advice I recieved.
When you eat, make sure to include EFA's (essential fatty acids) such as those extra-virgin olive oil, fish oils, and flax-seed oils. I took to including olive oil in my morning breakfast oatmeal, which helped not only with weight stabilization, but also general satisfaction. (It actually tastes REALLY good.)
Drink a protein shake in the morning, and one close to when you go to bed. This will help prevent your body from consuming proteins you need as your body becomes catabolic from high activity.
Watch when you eat as well. Make sure that within half an hour of finishing ANY endurance-based activity you ingest a tasty snack containing a 4:1 ratio of carbohydrates to protein. (chocolate milk and cliff bars are pretty good) This helps your muscles recover, and restores glycogen in your cells for more energy later. About an hour and a half later, eat a well balanced, medium sized meal (450-550 kcal). Salads with grilled chicken, olive oil, egg, carrots, tomatoes, etc. work really well for this purpose.
After or during a resistance based workout such as lifting weights, take in a bit of extra protein (whey or soy protein work well, but can be overdone).
As for your weight lifting workouts... Are you completing high-rep, low weight, or vice-versa? High-rep, toning workouts tend to rely more on the same slow-twitch muscle fibers that may already be worked and in recovery due to your cycling and swimming. Try doing some 4-8 rep sets at slightly higher weights once in a while to help "bulk up" a bit. (obviously, not to hulk out, but just to add a bit more muscle. [img]cyclingforums.comwink.gifOpen ↗[/img])
Take a recovery day once in a while as well, as it often helps you mentally as well as physically. (Sit back and enjoy an afternoon at the movies or something, just give those muscles a break!)
Check out www.fitday.com for a great way to keep track of your caloric needs. I have found it to be a great help, and in conjunction with hard training sessions and correct eating, I've gained roughly twelve pounds of lean mass since last july.
Hope this helps