Unfitmother - I agree with what others have said.
Academic life always relies on being able to quote and build on he work of others. Proper referencing of those quotes and areas of other people's work you are commenting on is perfectly respected. However, what is not accepted is just copying large tracts of someone elses work and presenting it as your own.
Showing that you have read widely and considered many different points of view in a particular field is regarded as an academic strength. Copying someone else's work without acknowledging them is seen as being beyond the pale.
Sadly, plaigerism is very common outside academic life where some professional consulting firms, City analysts and jouralists often take other people's work and and present it without referencing the source. Often this is done out of ignorance and lack of training on 'fair use' and 'proper ethical' standards.
In my academic/profesisonal work, if I needed need to quote a sentence or even paraphrase of what another person has written, I would just put the author's name and year of publication in brackets afterwards and the full reference at the end of the piece of work. If I wanted to quote a whole paragraph I would indent the text and make it a smaller font with the author and year straight after and full reference at the end of the piece of work. It is quite OK to cut and paste a sentence or a paragarph to make sure it is a pefect quote - as long as the reference to the source is there as well.
What I have described above sounds like what you have done so I am sure you will be just fine. There is also usually an 'allowed' amount of cutting and pasting without proper references. I would not worry on this score either, even if you have not referenced properly, as you say it is just a few sentences.