Gerbils balled up in play pen

Hey so bonded some gerbils recently- both older whose brother died. We split cage method them and it went great. Very slowly introduced them. They have been sleeping together for a month. Recently we have been getting them out in the play pen.

They went from sleeping together today to balling up and fighting. Very quickly separated them.

Now I dont know what to do. Is it the stress of the play pen? Can we put them back together in the cage?