Hives appear out of nowhere, even when my diet hasn't changed nor the have i products i use. they last no more than 24hrs. what could cause this?

Uncertain: When hives appear "out of nowhere", it's a challenge to figure out the cause. OTC & Rx medications are s0metimes the culprit. Regardless of the cause, it's wise to take an anti-histamine to control the itching and prevent worsening. If you ever get swelling of your lips, mouth or throat or you notice wheezing, seek IMMEDIATE medical care. Consider seeing an allergist for skin testing.

Many things: Assuming this is new for your, the most common cause is infection (even instances you're not aware of) resulting in activation of immune pathways in skin that trigger mast cells. Usually, hives resolve and we never know the cause. For other causes: http://emedicine.medscape.com/article/137362-overview#aw2aab6b2b3aa. OTC Zyrtec (cetirizine) may help if no contraindications. Consult Allergist if need more help