if you lose weight really quickly and your skin becomes flabby as a result, there is a way to make it tighten again, right? like, will it naturally go back to normal, or is there like lotion for that or something?

Honestly it depends on how much skin youre talking about. If it was an extreme amount of weight that you lost and you were carrying it for long time ~ your skin actually loses elasticity and wont go back....
Vitamin B and Vitamin E are good to help your skin but you should maybe see your doctor and discuss the issue further.
Also:
According to a co-worker of mine who lost 85 lbs doing the Dr. Bernstein diet ~ he gives you a shot of straight vitamin B every week to help ensure that your skin doesn't become flabby after the weight loss. Apparently you can purchase the vitamin B shots at any pharmacy as well. Good Luck !!!

Keep on working out those muscles if you can turn it into muscle you can get rid of it but its tough!!!
Try these rememdies of mine great for firming, and youthful looking skin I use for that and stretch marks Also try exfoliating get yourself a loofah and bodywash scrub problem areas thoroughly than apply your mousturizer(exfoliation helps to revive skin and get rid of dead skin cells)
My homemade recipes
1 cup olive oil
1 cup coconut oil
2 oz vitamin e oil (10-12 capsules)
6 vitamin a capsules
Heat on low to melt coconut oil and blend ingredients store in fridge (for lotion bar as it will harden) or in cupboard for your basic oil
6-8oz cocoa butter
6 oz almond oil
4 oz coconut oil
2 oz vitamin e oil
6 vitamin a capsules
Heat on low to melt coconut oil and cocoa butter blend ingredients store in fridge (for lotion bar as it will harden) or in cupboard for your basic oil
Use any of these ingredients combined or seperate