Have you been checked for functional B12 deficiency? I'm still recovering from a month's stay in hospital as a result of that. For me the parasthesia started in the exact same way and gradually spread up my legs to lower abdomen and my arms. I was effectively paralysed for the majority of that month. There are other auto immune deficiencies that can also cause similar issues, Guilles Barre Sybdrome for example. Your body's immune system attacks the metabolic processes that convert B12 into the form the body uses for nerve maintenance and protection. I was using many RCs on an a semi regular basis but nitrous oxide really hammered the final nail into the coffin. I'd advise seeing a doctor for blood tests to see how your B12 is doing. Functional B12 deficiency can even show high levels of B12 in the blood but your body simply can't convert it into anything useful. Push for more comprehensive tests.
I wouldnt advise ignoring parasthesia and hoping for the best which is what I did. Get some blood testing done and in particular push for checks against functional B12 deficiency if they tell you levels are normal. I'm not trying to scare monger but when the symptoms first started for me I googled them and reached the conclusion they would pass fairly rapidly and it was all quite benign. It turned out to be quite the opposite. The neuropathy slowly spread to the point that I was lucky to have been admitted to hospital on they day I finally went down there. I was a gnats hair away from it spreading to my lungs and being placed on a ventilator.
Not that this is that likely to happen to you, the condition that my B12 issues triggered is called Transverse Myelitis and is quite rare. That said it scared the fuck out of me, I'm still rehabilitating now, two months since being out of hospital, extensive physio work, living with my parents again and out of work. I would never take parasthesia symptoms so lightly again. I'd strongly advise to go and get yourself checked out if you haven't done so already. On the plus side the slower the onset of the symptoms in cases like mine the higher the chance of a full and relatively speedy recovery.
As I said my condition is quite rare so it's unlikely to be the cause of your issue but I thought it worth a mention; better safe than sorry and all that!
Wishing you all the best, hope it clears up one way or another. I can sympathise with the discomfort!
MrA