Possible to have "weak" blood vessels? Sometimes after working out I get a bad bruise on leg which fades to a little knot before slowly going away.

Likely Hematoma. You may be working out too hard and rupturing your blood vessels into the exercised muscles. Take it easier, and build up to heavy reps rather than jumping right in. Now, it is possible that your vessels break easily (are "weak"), or your blood is "thin" (you don't clot normally). Avoid aspirin, Ibuprofen and anti-inflammatories (unless you get a hematoma, when they are helpful); try vitamin c.
Causes as below. Trauma, prlonged coughing or vomitting, allergies, taking blood thinners, surgeries, blood disorders, including problems with blood clotting such as hemophilia a or hemophilia b. Blood diseases- leukemia liver disease- cirrhosis lymphomas. Disorders in which bone marrow cells grow at an abnormal rate nutritional deficiencies in vitamins c, k, b12, or folic acid. Infection in the bloodstream. Lupus etc.