Hi kathy normal nipple pain should last minutes while the blood flow returns to the nipple, the aching sometimes lasts for me upto 24 hours, but I do enjoy that but you should never feel anymore than a achey feeling for more than a few days.
I would suggest not leaving them on too long next time and see if you feel the same achey pain after.
I adore nipple play and own almost every type of clamp available, Im the worst as I do tend to leave mine on often all day as I love the pain of them (depending which type of course) each type causes a different feeling, so if your feeling in anyway uncomfortable stop till the ache goes.
Firstly can i ask; are you applying the clamp to end of the nipple or behind the nipple, as sometimes to keep clamps on its best to go behind the nipple (especially if you have pierced nips); but causes alot more pain and you have to be careful doing this, you can bruise yourself badly here?
These are great for beginners
http://www.lovehoney.co.uk/product.cfm?p=4952 as you can open the tweezers easily to fit.
These spread the compression across the nipple , but you cannot adjust the compression.
http://www.lovehoney.co.uk/product.cfm?p=1766&reviewpage=1#reviews
These clamps are adjustable but will pinch the nipple in a smaller surface area causing more pain,
http://www.lovehoney.co.uk/product.cfm?p=8041
These clamps are more like presses but cover a larger surface area of the nipple, so less pain unless you really want to; as you can close these completely ( I mean completely flatten the nipple, very painful and not advised).They are also awkward to get on initially.
http://www.lovehoney.co.uk/product.cfm?p=9818
And these are the ultimate as some are not adjustable and not recomended for beginners.
http://www.lovehoney.co.uk/product.cfm?p=1769
But my all time favorite nipple toy has to be this nipple kit with 4 different size rings, they such and increase the size of your nipple, increasing your sensitivity and then you can add the small ring (of your size, no smaller please), then you can wear these out.
My nips feel incredible after being sucked with this, a must try!!!Its safe too as if you suffer with achey nips you can just use the little pump leave off the ring as you still get the same feeling and sensitivity; the rings are a bonus.
http://www.lovehoney.co.uk/product.cfm?p=9831
Sorry this long i just felt each type needed slight explanation. I will be doing an article soon explaining all the different types in detail.