Upvote:2
There are many agencies in the US that track immigration records, listed on the USCIS website. Your best bet seems to be requesting your Entry and Exit records from the CBP (see http://www.cbp.gov/site-policy-notices/foia/faq-foia). Note that according to the first link I provided, "CBP does not have records on the entry and exit of persons arriving or departing the U.S. before 1982", but if you're interested in only 20 years it should be enough.
Another relevant link: http://www.cbp.gov/site-policy-notices/foia/records