As said mate the joists you have used do look a bit on the small side that sort of span should be 45mmx150mm (6x2) minimum, I know you said it's only for storage but the trouble with storage space is you keep finding things to put up there "you should see my loft it's like Jewsons back yard" plus you will be walking on it as well.

you could halve the span by running a 7x2 joist parallel with the last joist at a lower level then Tee of that one with a hanger and go underneath you're existing joists, but by the looks of the photo you probably won't be able to get a fixing for a joist hanger on the wall with the blocked of doorway

But see how you go you could always do this later if it looks a bit dodgy when you get some weight on it (but don't forget to take the stuff of it first)