If I sublease in a co-op and just signed a new sublease 2 months ago, do I have to leave before my lease expires since the owner has now decided to sell?

You description states that you signed a sublease for a coop. Although this constitutes a subletting of the coop unit by the unit owner (known as the proprietary lessee), the signing of the sublease creates a landlord-tenant relationship between the unit owner & the subtenant (subletting ordinarily requires the approval of the coop board). The sublease is generally subject to the terms of the proprietary lease between the unit owner & the coop corp, any rules & regulations & the bylaws of the coop. If you are not otherwise in violation of any of the terms of your sublease, which might give the owner (sublessor) grounds to start an eviction proceeding, unless there is a specific lease provision which allows the unit owner to exercise the right to early terminate your sublease & subtenancy upon notice, the unit owner is bound by the terms of your signed sublease. 

As a practical matter, if the unit owner has a purchaser for the unit, the owner can try to negotiate an early surrender of the sublease with the subtenant. Of course the purchaser will want to purchase a vacant unit, so they can move in after closing. Generally the purchaser's attorney will make sure the contract of sale provides that the unit be delivered vacant at the time of closing, so the purchase can take immediate possession.

I suggest you consult with an attorney who practices in this area of law and have them review any relevant documentation.