The main questions to ask your self are:
How quickly after injection did they appear? A histamine reaction will usually show up within minutes of injection. If that is not the case and it showed up for example, 24 hour after the injection, you are dealing with a different animal. See a doctor.
Do they itch to the touch? Is there any pain/swelling? a histmaine reaction will not be painful and swell like a missed shot.. it will be more itchy with raised splotches. If there is pain/swelling see a doctor, ESPECIALLY if this is increasing after 24 hours and not decreasing.
How long have they been there? A histamine reaction should go away pretty quick.. like an hour or so. If what you have seems to be worse in an hour rather than better.. see a doc
How are they appearing in relation to your injection site? A missed shot for example will be a lump/pain right at teh site. Histamine reactions will have raised splothces (not swollen with pain) around the injection site and within a 12" radius or so (IME).
If there is any pain, or the reaction seems to be increasing with time and not subsiding, DEFINITELY see a doctor. If you find your self very concerned.. listen to your self, see a doctor.