The quick response is that you should report your work activity to the Social Security Administration. You can not assume that the filing of tax returns to the IRS meets the SSA work-reporting requirement. At this time, you can report your return to work on the SSA.gov website, or you can make an appointment to discuss your situation at your closest SSA field office.
However, to better answer your question, it would be important to know your age, and whether you are receiving disability insurance benefits, SSI disability, or perhaps benefits on a parent's SSN as a Disabled Adult Child.
Assuming you are receiving Disability Insurance Benefits on your own SSN; or Disabled Adult Child Benefits on a parent's SSN; it is possible that you can receive continuing benefits as a self employed individual. You should look at the 3 "tests" for evaluating work by a disabled individual under 20 CFR 404.1575, linked below. Note that the standard for disqualifying substantial gainful activity in 2013 is $1040 monthly. The regulation also discusses your question about how SSA looks at "averaging" your monthly income over the entire year.
It is also very important that you review the rules for "Impairment Related Work Expenses", or IRWE, reported in 20 CFR 404.1576, also linked below. SSA recognizes that people with disabilities who work may have additional expenses caused by their impairments, which can be deducted, in addition to standard business-related deductions, in determing whether the net income you draw from your work is greater than the disqualifying $1040 monthly income level. Examples of IRWE include disability-required modifications to a residence or automobile, or medical device or medical treatment costs, including prescribed medication, not covered by insurance.
Take a look at the links below so you can familiarize yourself with the rules for people with disabilities and permissible self employment, then be ready to report your work to SSA.
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The application you signed when you filed for SS benefits had language in it that you agreed to notify SSA if you returned to work, or got workers' comp benefits, and a whole list of other things. So, I agree with my colleague - you need to immediately notify SSA what is going on.
There is a chance you have been overpaid, and it is better that you tell them what is going on than they find out when the IRS reports what taxes were paid - if that happens it may result ina criminal investigation.
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