Hey Ruth! If there are absolutely no signs of bed bugs (upon close inspection), its unlikely that they are the cause for the bite marks. I recommend taking another look for dark brown marks (dried blood) and other signs of their presence. Regardless, if you want to kill any bug on that mattress, just get a handheld steamer and blast it. The heat will kill everything and its completely natural. Hope that helps!
The size of bed bug bites varies with a number of different factors. Bed bugs inject an anti-coagulant along with their saliva when they pierce the skin to take a blood meal. This anti-coagulant is mostly responsible for how a person reacts to the bite and determines the size of the bug’s bite. Since people will have various sensitivities to the bed bug’s bite, the size of the bite will vary, as well. Another factor that influences the size of a bed bug reaction is the number of times a person is bitten. Bite reactions of people bitten many times are also variable, and their response may be either more or less intense as the number of bites increases.