Hair loss or just normal pattern

Diet change can cause problems with hair especially when you drop weight very quickly. However 5-7 strands in the shower really isn't a large amount. During the day the scalp sheds an average of over 100 hairs, most of the time, particuarlly when your hair is shorter, you don't notice this shed. Scrubbing harder is likily causing what you are seeing as an increase in shower shed. If your hair were falling from normal activities - for instace you find it on your pillow in the morning, its sticking between your fingers when your run them through it, gentle activities eseentially, that would be cause for concern. It sounds like your body is just recovering from your weight loss and you are being harsher on your hair while shampooing. Try adding a conditioner to your shower routine to see if that helps. Also are you dying your hair any? That can cause both breakage and dryness particuarlly with hair is is very dark because it takes stronger chemicals to change its color. 

hair fall isn't CAUSED by longer hair - rather it is more noticeable.
It sounds like your problem is primarily dye and bleach causing damage, dryness, and brittleness - diet change might be causing a slightly elevated fall. Try adding a multivitamin daily, and take better care conditioning your hair regulally, it dosent need to be shampooed daily either. That should help get your folices stronger (the vitamins) and help with the breakage (the conditioning).  Your loss sounds fairly minimal and like you should be able to take measures to help solve it yourself, but if you feel very concerned you can also go to a dermatologist.