SDS PAGE gels gives only an approximate MW, which sometimes is ambuiguous, like when a disulfide bond is present. You can use ExPASy ProtParam or pI/MW tool for accurate mass prediction. Use MALDI TOF for accurate mass determination.

Hi surya,

Molecular weights of protein differ according to species of your interest too, than your reference species. pI/MW is quite accurate for calculating protein mol wt. After this firstly, you can refer to antibody reference sheet provided by antibody provider for expected band size. and secondly, you can use antibody specific to your tag (as your protein has tag) to have a cross reference with your protein specific antibody.

Your protein may have some posttranslational modification. For example, N-glycosylation will significantly add molecular weight. One N-glycan almost add 2 kb. 

If a protein has an unusual amino acid composition, particularly with respect to charged side chains, it may run anomalously on SDS-PAGE.

it is an issue with SDS uptake of your protein. If it ist too acidic or  has some post translational modifications like glycosylation, it has less uptake of SDS that means it runs higher than expected 

I agree with everyone's post before mine. One thing to think about is the sequence the antibody binds to. You might be seeing a splice variant of the protein of interest. One possible way to test this is to use a different brands antibody. That might be costly, but I found that a variant of my protein of interest was coming up on my westerns.

Thanks everyone for your kind suggestions