Is there any special reason why you think you could be having syphilis?
Did you have unprotected sex or oral unprotected sex with a person who could be having this sexual transmitted disease? If you believe this is the cause of your bump under your tongue you should see your doctor as soon as possible.
Otherwise this bump is usually called a mucocele.
A mucocele should go away by itself after some time and no special treatment is needed. This is basically a swelling which is caused by saliva. This saliva is trapped within a blocked salivary gland and the result is mucocele.
Keep us posted about your condition and I hope that this will go away by itself.