Is this normal? (New DM)

One monster versus a full party will never end well for the monster. You need to keep the party on their toes with more than a single thing, or the fight will be ended fast. 

• Environmental effects: difficult terrain, hazards (fire, acid, spikes), obscured area (smoke clouds, walls/ruins), make the area more than a flat map.
• Minions: Draw off the fire and also make the characters feel heroic as they obliterate small enemies
• Reinforcements: If it's going to easy, introduce some new foes, and try to catch the characters off guard.
• Smaller more random encounters when they're weakened to drain them a bit.

Numbers on a page can get boring after a little bit, and it's okay to spice up a module because it will usually make things more fun for you and your players.

Quote from WolfeCriedPup >>

One monster versus a full party will never end well for the monster. You need to keep the party on their toes with more than a single thing, or the fight will be ended fast. 

• Environmental effects: difficult terrain, hazards (fire, acid, spikes), obscured area (smoke clouds, walls/ruins), make the area more than a flat map.
• Minions: Draw off the fire and also make the characters feel heroic as they obliterate small enemies
• Reinforcements: If it's going to easy, introduce some new foes, and try to catch the characters off guard.
• Smaller more random encounters when they're weakened to drain them a bit.

Numbers on a page can get boring after a little bit, and it's okay to spice up a module because it will usually make things more fun for you and your players.

Loved your message! You gave me several good ideas for my next combats.

Thank you so much!