Hi and welcome to HCM.
If you noticed this suddenly than it can be insect bite or you just sit on something. If this is there for some time then it can be any subcutaneous lesion such as abscess
etc. The most important is to evaluate is this infalmmated or not. If there is no redness
around it and this isnt painful then you dnt have to worry just wait and see will it progress in size and shape. If you notice some of these signs then visit doctor. Wish you good health.