What is the fastest way for me to lose acces weight?

im 5'6 and weigh about 142 pounds. im very athletic and toned and my stomach is some what flat but i want it to tighten up. ive heard muscle weighs more than fat so idk. is there any FAST FAST FAST way to lose about 10 pounds not tht much. my doctr says if i lose much more ill be under weight. i just want to be healthy body weight and feel good about wearing a bikini. so what could i do? oh and is hula-hooping a good way to flatten/tighten my stomach? what foods would help? please and thank you(: