I've been teased for bruising when the wind blows. I never associated it with low Vit D ... but now that I think about it, it's been better in the last few years, which has been the amount of time I found out I had low D and began to actively manage it.
I think that when I get to my lightest, for a time, I get more bruises but I think it has much more to do with my mind readjusting to the size of my body and banging into things, than to any physical deficiency.
Having said that, there are a few illnesses that can cause a person to bruise more easily, so it worth getting some blood tests run.
When I was 15 I caught mono - prior to that, I didn't bruise ever and after I bruised easily and they were painful, swollen bruises too. Especially on my shins. I'm not suggesting you have mono, just that physiological changes can definitely effect how you bruise.
"A mistake is not failure but feedback. -Rod Gilbert"
You need to let your doctor know. Most of the time it is nothing but a simple blood test will rule it out. At your age(mine too) there are some illnesses that are more prevalent and losing weight will not cause you to bruise more. I'm an Rn and you need to let your doctor know and get some blood work.