21female painless invisible lump on armpit, doesn't bother me to much been there for a few months can't see it but feel it what is it cancer?

You need. To have an expert feel it, measure it and decide whether more or nothing more be done.
See your DR for. An accurate diagnosis. It appears like it's an axillary lymph gland. Feel for any breast lump. Cancer is uncommon among young individuals unless there's a strong family history of it. Take care !