It could be enlarged lymph node. Especially since you have described it as hard.
Next Steps
You need to get a sonography of the neck, to see if there are any other enlarged lymph nodes which you are unaware of. also, it would tell us the status of
thyroid size, presence of any cysts etc.
Health Tips
The cause can be as trivial such as infection of the tip of tongue, or serious as malignancy. so get yourself investigated soon