I cut my forearm with a blade, deep and after 2 years the scar buldges when I tense my arm, the lump is very soft?

Normal. If your wound was deep and the scar that formed is wide, the area of scar is not attached to the underlying tissue the same way as the original skin. When you tense your arm the increased internal pressure causes the area of scar to bulge out. This is normal and no cause for concern.
Scar. you may have formed an inclusion cyst beneath the scar. It should not cause you any problems but can be removed if necessary.
Probably ok. Sounds like you may have some scar tissue you from the injury you suffered from two years ago I don't think this is anything to worry about but if you are worried about it from a cosmetic appearance standpoint you can always have your doctor refer you to a plastic surgeon.