Too high AB concentration, phosphorylation and other modifications, protease activity, alternative splicing, epitope present on other proteins, cross-reactivity of antibody with other proteins of unrelated sequence, signal by secondary AB, probably many more possible reasons.

Include as many controls as possible.

Is the finding reproducible?

How much volt did you use and what is the type (hand-cast or pre-cast) and the concentration of your gel?

I start with 100 volts and go to 200 after 5-10 mins

In addition to what was mentioned, I think the most probable cause is the high voltage you used, try to start with 50 volt for 25 min this would allow the bands to stack together and then proceed with 150 volt.