I have been getting 7-8 hours of sleep a night but for about a week i have been feeling tired all day even after having coffee. what could cause this?

Many reasons: One common cause is obstructive sleep apnea. There are many reasons, however, that you can be tired such as low thyroid hormone. You should see your primary for this

Sleep apnea: Excessive daytime hypersomnolence, sleepiness, and restful sleep, increased weight gain, with neck circumference > 16 inches in female, >17 inches in male, and Epworth Sleepiness >10 is indicative of underline sleep disturbance, such as untreated obstructive sleep apnea. Other causes of sleep disturbance, and excessive daytime somnolence need to be considered, includes: narcolepsy, hypothyroidism

Poor quality sleep: Even though you sleep 7-8 hours, the quality of your sleep may be poor due to frequent arousal during the night. Obstruction of the upper airway may cause this, or it may be due to your environment. Make certain your bedroom is cool and free from disruptions like noise or pets. If the problem persists a visit to a sleep medicine specialist may be in order.

Fatigue: fatigue has many causes and if you are getting adequate sleep then your fatigue may be related to things such as medication use, depression/stress or other conditions. If it persists best to see your doctor to get checked out to make sure nothing serious is wrong. good luck1