You need to know the actual fare class of the Economy fare you saw. K, L,T, S, etc. Then you can see if the fare actually went up or if the cheaper fare bucket just sold out. It's worth monitoring with something like Expertflyer to see how much of a particular fare bucket is left, if it wss sitting at L2 for example then may be a good idea to book it. If it's at L9 then you're safe to wait and see if general fare prices are adjusted down.