This happened with my DD when she was nearly one. She learnt to pull herself up and started to stand up in her cot, screaming her head off during her daytime nap and occasionally at night.
The first time she did it, I thought there was something seriously wrong and went in, picked her up, checked her nappy etc. She was fine so I put her back to bed and she carried on screaming for another 10 mins. As it was day time, I gave up and put her in her pram and took her for a walk and surprise surprise, she fell asleep.
She didn't do this at night, but the next day for her nap, she stood up screaming again. I managed to leave her for about 10 mins but she was not going to stop so I took her for another walk to let her sleep.
By the 3rd day, I was fed up of taking her out of her cot, so I left her there and when the screaming became unbearable, I went in, laid her back down on the bed and patted her until she calmed down. After 5 mins or so, she started to fall asleep so I left the room and she stayed sleeping.
On the 4th day and 5th days I repeated what I did on the 3rd day. By the the following week (after about a week of this screaming), she suddenly stopped standing up in cot (I peeped in on her) and she fell asleep after about 10-15 mins of screaming. I figured that as she was not standing up and injuring herself, I'd just leave her to scream until she ran out of energy.
She has screamed and stood up a couple of times at night too and I did the same thing as during the day i.e. go in when it gets really bad, lay her back down and pat her til she is calm and quiet. Then I leave the room before she is completely asleep. She has cried out again or wailed in protest, but generally has fallen asleep without screaming or standing up.
I really think it is a phase they go through. DD is now 15months and no longer stands up. She usually screams for 5 mins every time I put her to bed and then falls asleep. I just accept that she will always protest loudly before falling asleep.