In the space of a couple of hrs, i suddenly have a lump underneath my big toe. it's soft, not really painful but kinda large. i feel it when i walk ?

See a doctor: a lump underneath your big toe, kind of soft, but large. without a picture my guessing list could be enless, from lipoma, to a sebacious cyst, to a ganglion cyst, etc......... if you are concern about it, see a foot doctor.

I would tend: To think it was there and that you just noticed it. Lumps generally just don't come out of nowhere. In either case if it did nor did not i would recommend you see a foot doctor and let them examine you and tell you what it is. If it is a lump and they are not sure it should be biopsied.

Mysterious Lump.: Considering the time frame of a couple of hours and assuming that this lump has not been there all along, without being noticed, there are few possibilities. A leaking blood vessel, filling the available tissue space with fluid. This leak could be caused by a low grade trauma un noticed by at the time. A bite with insertion of foreign proteins could cause a quick lump to form. Ice it and see.