The old Assessed Income Period

I have a situation probably a little bit different than the norm.  I am originally from the USA where I married an American man who went on to be an actuary.  He made good money.  We then got divorced.  I then moved back to the U.K.  When I was 66 years of age, which back then was the U.S. retirement age,  I applied for a U.S. Social Security Divorce Spousal Benefit award,  The rule is that if you are married to a person for more than ten years, then get divorced but don't remarry, then you are entitled to claim this benefit on either your record or your ex-husband's record - whichever is better for you.  I have recently had the Divorce Spousal benefit award reviewed by a tax account who said that it is akin to alimony and is not taxable.

I was in an Assessed Income Period from 2013 to mid November 2018.  As many of you might know or remember that when one is in the AIP you were not required to inform DWP about your income, savings and investments.  Just before mid November 2018 DWP 'reminded' me that my AIP was coming to an end.  In mid December 2018 I informed DWP of my income.  They have now accused me of not telling them about my money; they are saying that I have a foreign pension that I never told them about.  They now claim that I owe them £2,300 +/- because of supposed Pension Credit paid to me from 2013-2018 and have started to claw back this money by taking money from my very small UK pension (I worked here as a young adult and then returned to the US when I was 21).  

My question is this:  since I did not have to tell them of any income, savings and/or investments during this five-year period then how can they require me to pay it back?  

They also stopped my Severe disability premium because they claim that my adult disabled son came here to look after me.  How can my son, who has Schizophrenia (and now Adult ADHD) come here to look after me?  I provided them with a psychiatric report which his UK psychiatrist wrote along with many other documents showing that they are wrong.  

This is the first time I have posted here, although I have been getting your emails all the time.  Are there any lawyers out there?  I have just finished making copies of documents both to PIP (which my son is now on) and to DWP.

Thanks for any help anyone can give me.

Debbie Gillies Rea