My doctor said it's normal but I'm wondering what causes that?!
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This is bruising. It'll go away with time. I'm sure you're aware but here are some tips to care for your stitches (from WebMd):

--To reduce the risk of infection:

-Do not wash the area for at least 24 hours. Depending on the type of surgery you had, you may need to wait longer.
-Follow your doctor's instructions exactly.
-Look at the incision every day, checking for signs of infection (see below).
-Change the dressing as your health professional recommends.
--Do not:

-Scrub or rub incisions.
-Remove the tape strips (such as Steri-Strips) from incisions.
-Use lotion or powder on incisions.
-Expose incisions to sunlight.
-Take a bath unless you can keep the incision dry. Take showers until your doctor says it's okay to take baths. Before you shower, cover the dressing with a plastic bag or use another method of keeping it dry.
-You may notice some soreness, tenderness, tingling, numbness, and itching around your incision. There may also be mild oozing and bruising, and a small lump may form. This is normal and no cause for concern.

Source(s):
http://www.webmd.com/a-to-z-guides/incis...