Thanks for asking the query,
Blood clot at the bottom of the tongue which is painless can be a ranula
which is formed by obstruction of the salivary gland
Sometimes it may rupture by itself.
If it becomes secondarily infected can lead to pain.
I would suggest you to visit a Oral Surgeon and get the lump excised, even after cure chances of recurrence are high.
Also get your blood sugar level analysed before procedure.
Maintain a good oral hygiene
, use lukewarm saline and antiseptic mouthwash rinses.
Hope this helps out.