Is this correct way to setup DB Infrastructure with repository pattern and Dapper

So, I am using Repository pattern with Dapper and ADO.NET. I have my based DB infrastructure setup as follows:

public class ConnectionFactory: IConnectionFactory
{
    private readonly string dbConnectionString = Convert.ToString(ConfigurationManager.ConnectionStrings["MyDBConn"].ConnectionString);
    private MySqlConnection _dbConnection;
    public IDbConnection GetDBConnection
    {
        get
        {
            if(string.IsNullOrEmpty(dbConnectionString))
            {
                throw new ArgumentNullException("DB Connection string received a null argument!");
            }

            if (_dbConnection == null)
            {
                _dbConnection = new MySqlConnection(dbConnectionString);
            }

            if (_dbConnection.State != ConnectionState.Open)
            {
                _dbConnection.Open();
            }

            return _dbConnection;
        }
    }


    public void CloseDBConnection()
    {
        if (_dbConnection != null && _dbConnection.State == ConnectionState.Open)
        {
            _dbConnection.Close();
        }
    }
}

And I have a Repository class that uses this infrastructure:

public async Task<IEnumerable<UserInformation>> GetUserInformation()
{
    IEnumerable<UserInformation> list;
    string querystring = "SELECT * FROM users WHERE valid=1;";

    try
    {
        list = await SqlMapper.QueryAsync<UserInformation>(_connectionFactory.GetDBConnection, querystring, param, commandType: CommandType.Text);
    }
    finally
    {
        _connectionFactory.CloseDBConnection();
    }

    return list;
}

For reading my IReader and converting to a dataset, I do the following:

public async Task<DataSet> GetOrderDetails(int oid)
{
    DataSet dataset = new DataSet();
    string querystring = "select * from orders WHERE OrderId=@oid";
    DynamicParameters param = new DynamicParameters();
    param.Add("@oid", oid);

    using (var list = await SqlMapper.ExecuteReaderAsync(_connectionFactory.GetDBConnection, querystring, param, commandType: CommandType.Text))
    {
        dataset = helper.ConvertDataReaderToDataSet(list);
    }

    return dataset;
}

Here ConvertDataReaderToDataSet is a method that only converts the IReader to a Dataset.

The problem that I having is that I am getting the following error after a while (from logs):

2/10/2021 12:37:44 AM
Type: Exception
Error Message: error connecting: Timeout expired.  The timeout period elapsed prior to obtaining a connection from the pool.  This may have occurred because all pooled connections were in use and max pool size was reached.
Method Name: GetOrderDetails

I have researched this error and it suggests a memory leak. Can you guys help me identify what is the problem and review my DB infrastructure setup?

Edit: After a discussion, I have changed the implementation to this:

public async Task<IEnumerable<UserInformation>> GetUserInformation()
{
    IEnumerable<UserInformation> list;
    string querystring = "SELECT * FROM users WHERE valid=1;";

    using (var conn = new MySqlConnection(dbConnectionString))
    {
        list = await conn.QueryAsync<UserInformation>(querystring, commandType: CommandType.Text);
    }

    return list;
}