Your heart beats fast when you stand up from sitting or climb stairs This started only in the last month so What might be causing it?

Postural orthostatic tachycardia syndrome (POTS): POTS is a condition that causes your heart rate and blood pressure to increase when you stand up. This can cause symptoms such as lightheadedness, dizziness, and fainting. POTS is more common in young women and is often triggered by certain events, such as illness or injury.

Iron deficiency: Iron deficiency can cause anemia, which is a condition in which your blood doesn't have enough red blood cells to carry oxygen throughout your body. This can lead to symptoms such as fatigue, weakness, and shortness of breath. Iron deficiency can also cause your heart to beat faster because your body is working harder to pump oxygen-rich blood to your tissues.

Dehydration: When you're dehydrated your blood pressure can drop, which can cause your heart rate to increase to compensate.

Anemia

Other underlying heart conditions: In some people, rapid heart rate upon standing up may also be a sign of certain underlying heart conditions, such as dysautonomia.

If you're experiencing fast heart rate when you stand up, it's important to see a doctor to determine the cause. Treatment will depend on the underlying cause.