Busy Lizzie leaves curling up.

I planted busy lizzies outside in pots in mid April. The leaves have shrivelled and become discoloured and I fear that frost may have been the cause although I don’t remember any frosty nights since planting. I didn’t put any fertilizer in the pots,- just used the pots of potting compost which I used last year. If I should have used fertilizer or the like, could you please recommend a product as I am relatively new to gardening and seem to put in an awful lot of effort but don’t always get results!