Doctor here, please do not fear me.
Isolated swelling of one leg is a sign of deep vein thrombosis which can lead to pulmonary embolism which are kinda serious. You should have that checked out right now. Heart failure as another commentor said is unlikely since it's only single sided. Lasix is also not the treatement of choice for this since single sided fluid buildup is very rare.

Ofc. It could be totally benign, but from across the screen without further information this is hard to judge and it could be deadly so better be safe than sorry.