UK taxes/self assessment after moving back to us

US citizen here, been back in the US for nearly  2 years now and I just got a letter in the mail saying I owe 100 pounds for not filing out last year's self assessment. 
Before I moved in April 2014 I filled out my last self assessment( for 2014), paid my tax and closed my self employed status.  ( still have the confirmation email). Does anyone know why I would still owe tax?  Or who I could even contact about this?  I kinda don't really care since we're not moving back, but I might want to visit again one day lol..

Any advice? 
Thanks! 

You were flagged because you had previously filed SA, and since HMRC didn't get one for the 2014-2015 tax year, you wee presumed to be delinquent. The £100 is not tax, it's the penalty for not filing a return on time.

The letter surely has an address and references your NI or the taxpayer number. Write to them and explain your situation in full, and attach a copy of the email.

Will do!  Thanks very much...

Ideally you should have filled out form P85, which would have notified the HMRC that you were leaving the UK and would have stopped the liability for self assessment.