Company No Longer Honoring My Raise For The Last Three Pay Periods

I've had a side gig for a company for about three years now and, up until recently, my rate was $1,000 a month. Starting in February, a colleague left and I was asked to take on their duties, and negotiated a pay increase to $1,500 per month with the COO (who I report to directly). Recently, I noticed that I had not been paid in three months (there were three outstanding invoices for $1,500 each). I emailed the COO, who never replied, but I quickly got a response from the owner of the company. She told me that I had not been paid for those three months because the rate on my invoices ($1,500) did not reflect the rate I was hired at ($1,000). She said if there was an increase to my pay, it was never communicated to her or authorized by her. I replied and forwarded her the email where the COO had authorized the pay increase, and also an email the COO had sent her in March very clearly communicating this pay increase. She basically replied saying again that she wasn't made aware of the pay increase, and then went into all the things I was doing wrong in my role (which is wild because in the three years I've worked there, she has barely ever provided actionable feedback or even had a conversation with me). She said she "temporarily sent me pay at my usual rate until she figures this out," which she did ($3,000 instead of $4,500 it would be with the new pay rate).

I'm curious - do I have a case here for getting this additional pay? Do I report it to the labor department? I just submitted my invoice for May's work, so presuming she pays me my old rate again, at this point, the owner will owe me an additional $2,000. I live in Illinois and the company is based out of Virginia, I believe. I can provide more details if need be, but hoping for some insight as I've never dealt with something like this before. Thank you!