Photo Exclusive: Inside J.Lo's Big, Glam Bash!

See how Lopez surprised her husband Marc Anthony on his 40th birthday

LET'S DANCE!

If the morning's triathlon wore Lopez out, she didn't show it. The singer-actress hit the dancefloor in her Marchesa dress. "I didn't feel horrible," she said. "Just a little tired. I had a little headache, maybe it was the relief of the stress."