If this concerns you, you should visit your doctor as soon as possible for a check-up, especially if the bubble hurts. 
WebMD? Never heard.
Really? This is happen after I get burn to recover for two weeks and now I got this? *groan* Well when I check on it, I saw yellowish with whitish liquid. Wait...I have infected? *groan* I need go over WebMD because I neer see it before. So I am going over there and see if they can help me out with this problem.Severe burns leave scars - if the burn has healed then it's probably scarified tissue. If it's an actual bubble with liquid in it then it's still healing, unless the liquid is yellowish or whiteish, not clear like it's supposed to, in which case it may have gotten infected, which would explain the red colouring often associated with inflammation. Also, this is a gaming forum, not WebMD.
Well I didn't feel hurt. Ok I won't touch it. It start this morning. How long it take for healing?
White or yellow liquid is probably pus and it's not uncommon to see it build up in a wound even post-recovery if it wasn't cleaned properly (sand, dust etc. are alien objects and covering them with pus is the organism's way of dealing with the pathogen) or if the tissue that's stuck under the recovered skin underwent necrosis (which means that it's dead, decomposing and a bacteria bomb which has to be dealt with). Keep the bubble clean with some medical alcohol, medical hydrogen peroxide solution or any other medical antiseptic/disinfectant, it might go away all by itself. If pus continues building up, you could try popping it with a sterilized needle and drain it, but that's ill-advised without prior consultation with a doctor.
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