I suspect what has happened is that you've connected everything up with brand new RO membranes and started producing pure water. But there's some kind of 'stuff' on brand new membranes which needs to be washed away, but since you've had this high TDS water going through your DI chamber, your resin has purified it and quickly exhausted itself; hence the later high TDS reading.
Disconnect the tube going into the DI and tell us what the reading that comes straight out of each RO membrane is.
I suspect all you need to do - if your TDS readings after RO are fine - is reconnect it to your DI chamber and change the resin.
If this is the case, in the future when you replace your RO membranes, just run it for a few hours before connecting it to your DI; just let all the water go down the drain for a while.