Sometimes that happens.  Just make sure you are using an alcohol wipe before, AND after each injection.  Sometimes if the needle is a little dull or you push it in on a bad angle, or bad spot you may draw a little bit of blood.  This is harmless in a subcutaneous injection.  The injection sight may bruise a bit for a few days.  Next time just poke around until you hit a spot that is somewhat 'numb' and inject there.  There are plenty of painless injection sites on your belly.