I Am Having Dizzy Spells When Standing Up And Weakness

I am having dizzy spells when standing up and weakness in my legs simultaneously. I am a heart patient having had a double bipass surgery 5 years ago. I am taking a daily medication of metoprolol and baby aspirin. I am also taking vitamins: fish oils, kyolic garlic extract and B12 on occasion. My heart Doctor is not contactable on weekends, unfortunately, but will follow up in the morning. These spells have occurred intermittently over the past two years but it seems to be happening more frequently during the last 6-8 months. When these attacks occur or when I can feel it coming on I drink several glasses of water and sit down and wait it out until I'm back to normal. Thank you for your advice.