Possible liposarcoma?

4 weeks ago a pain free  lump came on around my armpit within 3 days it was huge, and within 3 weeks its causing me issues with my  arm getting pain down my arm pins and needles numbness etc gp sent me for an ultrasound who then sent me for mri I had my mri last Wednesday and today I've been told I've been referred to a sarcoma clinic and should have an appointment within the week as they are concerned about the size its 10cm x 6cm apparently and deep in my muscle , place and how quickly its grown but surely if it was cancer wouldnt the mri pick it up there must be some different characteristics between sarcoma and liposarcoma ??? I've no idea what happens from here I'm panicking obviously and on top of the lump I've got nausea and fatigue... I'm guessing I'm likely to have a biopsy now but am I likely to have it removed and then biopsy as its causing me issues or have a needle biopsy  have to wait for the results and then have it removed?? Anyone else had any similar experiences at all can offer me any advice ?