ZDC Controller experienced a loss of separation event during very heavy traffic; holding; and absent supervisor and D-Side assistance.

Narrative

I was working overtime on an evening shift and working a combined sector with approximately 27 aircraft in the sector. Sector was getting out of hand and I could not find anyone to help or a Supervisor to split the sector. I was working arrivals into EWR and arrivals into LGA as well as TEB with a steady departure flow from the metro area climbing between the two arrival flows. All three flows were busy at the same time. I was put into a no notice hold for EWR. I had cleared three aircraft into holding at ARD VORTAC and amended the altitude of Air Carrier X to 11;000 FT and issued holding instructions. The pilot read back the holding instructions; but not the altitude. Just after the read back I took a land line call and a Supervisor from another area interrupted me to inquire about the reason I shut off his sector. I told him I didn't have time to talk with him and that I was busy. I observed Air Carrier X leaving his assigned altitude and descending into Air Carrier Y that was holding at 9;000 FT. I immediately instructed him to climb back to his assigned altitude and issued traffic. Factors that led up to the loss of separation: working combined sectors with a high volume of traffic with high complexity (the sector was red); no D-Side provided nor a Supervisor in the area to ask for a D-Side; distractions from another area Supervisor; no notice hold for EWR (less than three miles from holding fix); and frequency congestion. Recommendation; [we need to] staff to traffic. It seems lately we keep sectors open when there is a very low level of traffic/complexity if we have the controllers and yet when we are short on people we combine sectors and don't staff the D-Side even though the sectors are red. I clearly needed help and could not get it.