Very odd trade on GDAX yesterday

Hi, yesterday I wanted to sell my BTC on GDAX when it reached around 9700 EUR (11500 USD) price expecting a correction. Just as I was setting the limit the price started dropping so I decided to just sell on the market. The price already fell to about 9500 EUR when I clicked sell. Then the browser pretty much froze and then the chart got messed up as well as other areas of the display so seeing something odd is happening immediately I did rebuy, I didn't even see the price at the moment and then the GDAX page crashed. I couldn't reopen the GDAX page again but I was remotely connected to another pc so I tried and managed to reconnect and then I saw I had about 20%-25% more in BTC on my account but soon it crashed again.

When finally I connected about 20-30 minutes later, it said I sold at 7100 EUR (that is about 8400 USD) and bought at 7800 EUR! That is 25%+ less than the price I wanted to sell at! Did this happen to anyone and does this happen on other exchanges? I sent a complaint because this definitely doesn't seem regular, I lost a lot of money and I see this as an error in their system. If this is regular than it means any time you sell on the market they can set any price they want, just crash the market before processing your transaction and sell... I feel cheated. So to explain further this crash lasted only a few seconds, you can see on BTC / EUR chart on GDAX a short spike down to about 7100 EUR (in USD the spike was much smaller) and back some time before the regular correction started and lasted few hours. Any thoughts?