I cannot straighten my left arm totally and have a hard bicep muscle, no pain to touch ...doctor said it could be a tumour .should i be worried ?

Need more info: Need a better history and exam details. Age, medical history, injury, mass detected on exam, diagnostic studies xray/mri? There are many possibilities most of which are non-life threatening. Sarcoma, cancer of the muscle, is possible but very uncommon.

Tendinitis: Hard biceps muscle without mass probably not associated with tumor. The latter would produce a nodular lesion and not interfere with motion at elbow. An MRI and bx if needed would resolve the issue and if inflammatory in nature, anti-inflammatory agents would be prescribed.