Bruising can occur because you passed through a vein during the injection, causing bleeding under the skin aka a bruise. Many times newbie injectors will begin well placing the needle but will tense up while injecting. When the needle is inside you and you begin to tense up and contract the muscle you are shooting or you begin to move a little, you cause the muscle to move. This movement can cause small tears within the tissue around the needle. This will cause discomfort that will last a few days. My advice if you are to remain shooting yourself is to stand on the leg you are not shooting keeping the side you are injecting very loose. Inject the glute high just couple of inches below the waist and relax. A cool trick is to place the full syringe above a lamp for about 1 minute this will heat up the oil making it real easy to pass through the needle, causing less chance for you to tense up.
I think you should continue your gear. Deca is usually real easy and seldom causes pain and swelling after the injection. This is why I believe you are injecting improperly.
Hope this helped, and trust me it gets easier and you will soon be on your way to spot injecting.