I'm feeling very weak post Gynecomastia surgery...how can I feel better??

Feeling week after surgery can be due to calorie depletion.  Contact your plastic surgeon as soon as possible to rule out any serious problem.  

Liposuction is typically used liberally in most gynecomastia surgeries.  Swelling after liposuction can be tremendous.  The fluid in the swelling can dehydrate you significantly.  Drink significant amounts of salt containing fluid in the next 24 hours (like Gatorade).  Contact your surgeon ASAP to let him/her know what you are experiencing.  Best of luck.