I suspect 5 possible culprits:
1. If you are new to taking Lexapro, i.e. not past 4-6 week mark, then give yourself some time. It takes some time for your body to get used to antidepressant medication as I know from experience. It can take a full 6 weeks for your metabolism and digestive system to adjust to the new medicine. These are extremely powerful drugs. Never underestimate the effects they are having upon your body. They are rewiring your brain chemistry, and since your brain controls the production of hormones which in turn govern every internal mechanism of your body, you can't underestimate the need for your body to have time to adjust. They are affecting your body and making it more tired than usual. Antidepressants also give you "dry mouth", they dehydrate the body and you need to take in extra fluids at least for 6 weeks as your body adjusts to the medication.
2. You are working out a lot and hard. You are pushing yourelf through discomfort and aches and pains. I suggest you schedule 2-3 days of this week to let your body rest and fully recover before you start pushing again. You will not lose strength or cardio endurance in this time. In fact, you will come back stronger and more motivaated. Your body's strength increases after the workout, when you are at rest whioch is when it repairs itself. If you never rest or rest insuffuciently for your body's requirements, you don't see the muscle recovery and strength gains.
3. Possible dehydration, as suspected by Gina, and also due to lexapro, see number 1 above.
4. You may not be eating enough, due to nausea brought on by Lexapro. In fact, if Lexapro is still causing you nausea, then this lends support to my argument in number 1.
5. Weight trining can lead to water retention in the muscles as they repair themselves. When you start a new program of training, i.e. new to weights, or switching from endurance based program to a heavier weights, strength based program, your muscles will retain weater intitially. After a few weeks, this will right itself. It is a normal part of the muscle tissue building processs and nothing to worrry about. Extra rest, as in number 2 above, will help.
Take care! Mostly because of the Lexapro. When starting a course of treatment with an antidepressant medication I always take down the intensity of my workouts for several weeks and rest much more. I can build it back up afterwards without any of the problems you are describing here. In fact, I have just done it. Living proof!! So, go easy on your body.....
Clare