Both of you might have simply hit a tiny capillary, which caused the bleeding. I use insulin and you wouldn't believe the bruises I sometimes get. Also, you don't feel any pain when you hit a capillary so you don't know you've done it. If anything wet gets under the sensor, it does tend to fall off. So swimming can also be a problem. If you're having problems getting the sensor to stick on in general, there are couple of things you can do. You can add an adhesive barrier like Skin Tac. Basically, Skin Tac forms a barrier so it sticks to your arm and the sensor sticks to it. Also very useful if you're allergic to adhesive. Don't bother buying the special Skin Tac remover, it's just rubbing alcohol. Also make sure your arm is clean and oil free. So don't apply body lotion before you put on a new sensor. Wiping with rubbing alcohol not only sterilizes the area, it also removes any oil. And if the sensor falls off early, contact Abbott Diabetes and they will replace it. I can't tell you how many sensors Abbott has replaced over the years. Sensors falling off is so common that in Canada we now have a dedicated website to request a replacement so we don't take up staff time.