What type of carrier do you use? As your DD is getting heavier a one shouldered carrier may become uncomfortable and eventually cause pressure points. This would normally manifest in muscular or ligament pain though, it would be unlikely to cause a lump or swelling.

You said that the lump is over one of the vertebrae on the neck - does the carrier touch that part of your neck when you wear it? If you are wearing a one shouldered carrier (or any carrier for that matter) correctly, it shouldn't touch the back of the neck at all, you may find that reviewing the way you wear it will help.

You may also find that using a two shouldered carrier like a mei tai or SSC (soft structured carrier, eg, Ergo) distributes weight evenly and transfers more weight on the hips and less on the shoulders.

HTH, and good luck at the Dr!