I have already painted a canvas, how can I make it tighter as it has come loose?

I do alot of paintings straight onto textiles, so this means that i do not use proper canvas fabric and i do not paint it with gesso first. I paint straight on with acrylic paint. this time because I painted the whole of the canvas the fabric has gone baggy, is there anyway I can tighten it? I heard that spraying hot water can help a normal canvas, will it help? Thanks! www.victoriathorpe.co.uk