Is the gym warm ? Some times a warm room can make a person feel some what lethargic.
Can exercise invigorate a person ? With time, but it's not something that happens immediately and there will be days you feel tired. We all get them, even people who exercise regularly.
Are you drinking enough water ? Do make sure you're drinking enough water when the temperatures are warm inside and out. Being a little dehydrated could make you feel a little sleepy.
Of course, are you getting a decent night's sleep or are you restless ? If you don't get a good night's sleep the night before that too will contribute to a tired day. Are you under a lot of stress ? Being under stress can make a person feel tired or fatigued.