girlracer, you need to talk to your coach. he/she should be aware of how you feel (physically and mentally).

given that, it's completely natural to feel "slow" while you're lifting heavy weights. your coach is having you lift heavy weights so you can increase your strength and (later in your training) improve your power. but if you're lifting appropriately, your legs (and your whole body) will feel fatigued. that's okay.

remember, you can't sustain a high level of performance year-round. you don't need to be fast right now. when does your race season start? when are your goal races?