You don’t remember bumping into anything. Yet somehow, you have a shin full of bruises and a whole lot of questions. There are plenty of reasons for easy bruising, the discoloration caused by a trauma that ruptured blood vessels just under the skin. Check out the surprising explanations behind your sore spots and learn which ones are harmless and which ones need attention.

1. You take a daily aspirin.

It’s the most common reason why you’re waking up with random bruises. Aspirin, as well as blood thinners, work by disabling the platelets that cause blood to coagulate or clot. With thinner blood and less clotting cells, even a minor trauma will leave a mark.

2. It’s just your skin tone.

If you have fair, thin skin, you’re naturally more susceptible to bruising even from minor bumps and bangs. Those of us with more adipose or fatty tissue won’t see a spot from the same level of trauma. Another culprit: varicose veins, spider veins or naturally more prominent veins.

3. You need more citrus in your life.

True vitamin deficiencies are rare in the U.S., but they do become more common with age. In fact, the reason people put lemon and limes in their drinks is because vitamin C deficiencies are common in alcoholics. Vitamin C is important in wound healing and the production of collagen, an important structural component of skin. Signs you need more vitamin C (and a trip to the doc for a blood test): fatigue, depression, bleeding gums, swollen joints, nosebleeds and dry hair and skin.

4. You never miss a chance to sun yourself.

A long history of sun exposure (especially skin-damaging sunburns) will leave you with thinner skin – and more bruises – even faster. You lose collagen as you age, but the UV light damages it even more. While you can’t turn back time, you can prevent future sun damage (and, in turn, a future of easy bruising) with a good SPF.