After lying on floor on my front and leaning on my elbows, one elbow became painful, red and hot to the touch. what could this be?

Elbow pain: Those symptoms are characteristic of acute olecranon bursitis. Fluid accumulation of the elbow bursa usually follows. I recommend getting examined by a physician to rule out cellulitis which is another possibility &requires oral antibiotics. Taking anti-inflammatory medication like ibuprofen or naproxen can relieve the inflammation and pain of bursitis.